This study examines the evolution of Ella Fitzgerald's scat syllable vocabulary during a key developmental period in her career when she was recording for Decca Records. Between 1943 and 1952, Fitzgerald established the syllabic vocabulary that would serve as a defining characteristic of her improvisational style for the rest of her career. Fitzgerald is commonly praised as the greatest vocal improviser in jazz history, but while much has been written about Fitzgerald's melodic and harmonic approach to jazz improvisation, little has been written about her syllabic approach. Timbre and articulation are considered to be vital elements of any jazz musician's style; the study examines the changes in Ella Fitzgerald's syllabic approach through transcription and analyses of thirteen scat solos recorded during this time period, using scat syllable choices to discuss timbre and articulation. This analysis provides a model for further research of its kind, as well as informing historically accurate performance practice by both teachers and students of jazz singing.