This contribution explores the question of expressive tempo modifications as a function of textual content in Adelina Patti’s and Luisa Tetrazzini’s recordings of “Ah! Non credea mirarti,” from Vincenzo Bellini’s La sonnambula (1831). Their analysis made it possible to determine to what extent significant tempo modifications tend to coincide with those moments in the music, in which special emphasis is demanded by the lyrics, and meaningful changes are observable in the melodic-harmonic profile. Having defined the context and clarified the relationship between the lyrics and the compositional solutions underpinning them, the degree of tempo variability for each aria was determined by empirically measuring the crotchet beat lengths and by calculating the value of mean, median, mode, standard deviation, coefficient of variation. Preliminary results show that Adelina Patti’s and Luisa Tetrazzini’s renditions present tempo modifications which are quite consistent with and instrumental in the expression of their dramatic content, as recommended by the singing methods and treatises which had appeared in the course of the nineteenth century.