This dissertation investigates how W. A. Mozart applies the concept of key characteristics—the affective properties of each tonality—as discussed by three of his contemporaries, Johann Mattheson, C.F.D. Schubart and G.J. Vogler, to four soubrette and four prima donna characters from four of his late Italian operas: La Contessa and Susanna in Le nozze di Figaro; Donna Anna and Zerlina in Don Giovanni; Fiordiligi and Despina in Così fan tutte; Vitellia and Servilia in La clemenza di Tito. The analytical method of this dissertation provides a hermeneutical tool to search for meanings in Mozart's music. The application compares the libretto text and its corresponding tonal center with the description of key characteristics on a micro level, to reveal significant dramatic and practical implications from Mozart's key usage in his operas.