The years around the turn of the century were among the most productive in Sibelius’s career. The period 1898-1906 saw a rich output of solo songs: opuses 36, 37, 38, 50, plus four songs which he numbered among his opus 17. In addition to these collections, he composed several songs which remained without an opus number. With the exception of opus 50, these songs breathe the same spirit of Late Romanticism as the first and second symphonies (op. 39 and 43) and the violin concerto (op. 47). The majority of Sibelius's most popular songs are included in opuses 36-38.

The cooperation between Sibelius and his friend A. V. Forsman (later Koskimies) which started with the Degree Ceremony Cantata of 1897 continued during the years that followed with the solo songs Illalle (To Evening, op. 17 no. 6, 1898) and Souda, souda sinisorsa (without opus number, 1899). The text of the song Illalle (To Evening) has a double meaning: the poem is both a hymn of praise to eventide and a courtship poem, written by Forsman to his sweetheart, Ilta (=Evening) Bergroth. Souda, souda, sinisorsa is also a courtship poem. Sibelius composed both songs in the spirit of Finnish folk song. It may be for this reason that both Illalle and Souda, souda, sinisorsa have remained among Sibelius's most popular songs within Finland. Vilse (Lost in the Forest, 1898; words by Tawaststjerna) is a humorous song about two lovers misled by the echo. Sibelius later included it in his opus 17. The sea song Segelfahrt (words by Johannes Öhquist) was completed and published in 1899, but later it was almost completely forgotten.

Opus 36The five songs of opus 36 have been among the composer's most popular songs ever since their first public performances. They were composed at the turn of the century, 1899-1900, and they were published during the same period. Many of the songs convey feelings of sorrow, loss and death. Song no. 1, Svarta rosor (Black Roses) was the first poem by the Swedish painter and poet Ernst Josephson that Sibelius set to music. The conflict between the lyrical sequences in C major and the dramatic sequences in C sharp minor conveys the tragic atmosphere of the text with great intensity. There is also a reference to death in the opening of song no. 2, a setting of Runeberg's Men min fågel märks dock icke (But My Bird is Long in Homing); here, the rising chord progression resembles the introduction to The Swan of Tuonela (op. 22 no. 2). Song no. 3, Bollspelet vid Trianon (Tennis at Trianon; words by Gustaf Fröding) brings a glimpse of humour to the collection, but the Grave sequence at the end lays a dark shadow on this song as well. Sibelius wrote two settings of the poem by Gustaf Fröding, Säv, säv susa (Reeds, Reeds, Whisper). The first version followed the Scandinavian song tradition, but the version we have here (op. 36 no. 4) emphasises the dramaturgy of the poem: the peaceful andantino and molto tranquillo sequences provide a setting for the stormy poco con moto which describes the drowning of a young girl, Ingalill. The collection ends with poems by Josef Julius Wecksell. Song no. 5 is Marsnön (The March Snow), and no. 6 is Demanten på Marsnön (The Diamond on the March Snow). The second of these used to be one of Sibelius's most popular songs, but its old-fashioned drawing-room aesthetic has not lasted as well as Svarta rosor, or Säv, säv susa.

Opus 37The magic and colour of Romanticism reach their peak in the opus 37 song collection, written in the years 1901-02. The first song Den första kyssen (The First Kiss; words by Runeberg) presents a dialogue between a girl and the evening star. According to Erkki Salmenhaara this song is a fine example of the way in which the composer uses the interchange between harmony and melody in order to reflect the psychological content of the poem.

The same holds true of another song by Runeberg, the tragic ballad Flickan kom ifrån sin älsklings möte (The Girl Comes from a Tryst with her Lover; song no. 5), which may be one of Sibelius's most dazzling vocal dramas. The second piece in the collection, Lasse liten (Little Lasse), is based on a children's poem by Zacharias Topelius. Yet despite its simple melody, the song does not represent the Kinderlied tradition. The sunrise and rider theme of the third song, Soluppgång (Sunrise; words by Tor Hedberg) anticipates Night Ride and Sunrise (op. 55, 1908). The pearl of the opus 37 collection is Var det en dröm (song no. 4; words by Wecksell), which Sibelius dedicated to his favourite soprano, Ida Ekman. "Here you have my most beautiful song", Sibelius said when he presented the singer with the manuscript of the song. In this song, love is symbolised by flowers in the same way as in the flower- songs of opus 88 – the difference being that the romantic glow and the highly-coloured piano texture of Var det en dröm is absent from the later song cycle.

Opus 38
The Late Romantic songs which Sibelius composed in 1903-4 while he was writing his violin concerto (op. 47) mark a change in his development as a song-writer. They take a step towards greater complexity in the musical language, co-existing with greater economy in the actual expression and in the use of harmony. The first two songs of opus 38, Höstkväll (Autumn Evening) and På verandan vid havet (On a Balcony by the Sea; words of both songs by Viktor Rydberg) are among the most highly-regarded songs in Sibelius’s output. For the first song Sibelius prepared a complex nature tableau of several episodes. Here, the long-held chords on the piano and the recitative song part that soars above it depict Rydberg's autumn evening scene with restricted yet impressive means. The structure of På verandan vid havet is simpler than that of Höstkväll. The modulations divide it in three episodes in which the musical material is repeated. Various aspects of this work seems to prefigure the fourth symphony (which was written eight years later) including its overall sense of profundity, its use of chromaticism and its use of a tritone motif. Song no. 3 is the only actual nocturne among all Sibelius's songs, I natten (In the Night; words by Rydberg). Here, the unison melody on the piano and the mezzo voice of the singer are the means by which a nocturnal atmosphere is produced. The last two songs of the opus, Harpolekaren och hans son (words by Rydberg) and Jag ville, jag vore (words by Gustav Fröding) are not as strong or original as the first three songs. In 1904, Sibelius wrote the song En slända (words by Oscar Levertin), which he decided to add to the songs in opus 17 (as op. 17 no. 5). In this song the free expressionistic dialogue between voice and piano bears a stylistic resemblance to the songs in opus 38.

Opus 50Sibelius's only solo song collection based on poems in German, opus 50, was created in the summer of 1906. It does not continue the powerful original line of opus 38. Instead, the songs come close to the German Late Romantic lied tradition. It seems that Sibelius was inspired in this direction both by German poetry and by the German publisher of the collection. Lenzgesang (no. 1; words by Arthur Fithger) is full of vitality and integrity. It is quite different from the songs in praise of nature which Sibelius composed to Swedish texts – songs which can be serenely reflective or even devotional in character (cf. the songs Fågellek and I natten). The charming Sehnsucht (no. 2; words by Emil Rudolph Weiss) contains deliberate echoes of salon music – an extremely unusual feature in Sibelius’s songs. The melancholy song Im Feld ein Mädchen singt (no. 3; words by Margarete Susman), which resembles a folk-song, is followed by the erotically charged Aus banger Brust (no. 4; words by Richard Dehmel). In this, the vocal line is restrained at first despite a passionate arpeggio accompaniment. However, it gradually grows into an outburst of passion (“tutta la forza”). By contrast, as Erkki Salmenhaara has pointed out, both the atmosphere and the structure of Die stille Stadt (no. 5; text once again by Richard Dehmel) recall the song Der Wegweiser in Schubert's Winterreise cycle. The last song, Rosenlied (words by Anna Ritter), has been described by Gustav Djupsjöbacka as the scherzo of the collection. After the songs mentioned above, Sibelius composed the song Erloschen (words by Georg Busse-Palma). As with his previous opus, he offered the song to Robert Lienau, his publisher at the time. However, for some unknown reason the song remained unpublished for several years