In 1883, Wolfgang Amadeus Mozart composed two duos for violin and viola. It is said that Mozart's friend Michael Haydn had become ill, and was unable to complete a set of six duos for his boss, the Archbishop Colloredo. Mozart supposedly wrote his two duos to be passed off as Michael Haydn's work. That Mozart intended to write these two pieces in Michael Haydn's style is impossible, as there is a great difference in how the two composers treat the viola part. Mozart regards the two instruments as equals while Michael Haydn clearly writes a more soloistic violin line and a viola accompaniment. Michael's brother, Joseph Haydn, also favors the violin line in his duo sonatas for violin and viola. Mozart's treatment of the viola part was highly innovative for its time. Additionally, this dissertation researches the various problems in performance practice in regards to the two Mozart duos. These issues include tempo, dynamics, articulation, phrasing, vibrato, and the selection of editions. While exploring these matters in depth, the performer will find that in combination with careful research of documents and treatises by the likes of Leopold Mozart, nearly every aspect of performing these duos comes down to a matter of personal taste. It is in putting one's taste into action that one can find an authentic interpretation of these works.