Johannes Brahms was one of the first composers to recognize the viola's untapped potential, allowing the instrument to shine in his chamber music works. All in all, although Brahms's two sonatas in F minor and E flat major, Op. 120 were originally written for clarinet and piano, a special affection for the instrument, i.e. its beautiful, warm tone, he intended the same sonatas for viola. Studying both scores (clarinet and viola) I can notice that Brahms is aware of the full potential of both instruments, and accepting the nature of each instrument he composed two different versions. Each of them is different not only in tone color, but also adapted to technique, register, volume, vibrato... Those two sonatas are extremely valuable in the viola repertoire, which is the basis of my thesis, with the exception that I would like to look at the significance of the viola in general in Brahms's work.