The Op. 42 Notturno for viola and piano is an arrangement of the 1796-97 Serenade in D, Op. 8, which was scored for violin, viola and cello. But it was not the composer who reduced it to piano and viola here. F. X. Kleinheinz did the arrangement and presented it to Beethoven, who corrected it and approved it for publication in 1804. In the same year Kleinheinz also fashioned an arrangement of Beethoven's Op. 25 Serenade for flute (or violin) and piano, with the composer once more making emendations he deemed necessary for publication, at which time he assigned the opus number 41. There were other instances where Beethoven allowed another hand to transcribe his music: the Op. 104 String Quintet (1817) was an arrangement (now attributed to Kaufmann) of the Op. 1 Trio, No. 3, and the Op. 64 Sonata for Cello and Piano was an arrangement by an unknown party of the Op. 3 String Trio. And there are more such instances.
The Op. 42 Notturno is comprised of seven short movements. The first is a rather simple march of some appeal, while the second, an Adagio, is much longer and offers greater rewards. The ensuing minuet is attractive but proves to be an interlude between the two (more substantial) Adagios. The second Adagio alternates effectively with a contrasting Allegro molto Scherzo section. There follow an attractive Polonaise and a theme-and-variations marked Andante. A reprise of the opening march concludes the work. Overall, it is a delightful but still minor composition in Beethoven's vast output.
One of the key issues surrounding this work, as well as most of the Beethoven arrangements, is its validity in the new instrumental guise. By its very nature, this kind of composition becomes problematic, owing to the non-sustaining, harder tones of the keyboard rendering large portions of music clearly more suited to the sweeter, sustaining sounds of the strings. Thus, the Adagios in the work, for instance, are less effective than in the parent composition. In the end then, while this piece has value and will offer enjoyment to the listener, it cannot be considered the preferred version of this attractive score.