This paper examines, from a Thomistic perspective, the possible consequences of the twentieth-century reform of the ‘Kyrie eleison’ from ninefold set of invocations to a sixfold call-and-response structure. First, we present Aquinas’s distinctly Trinitarian exegesis of the ‘Kyrie’ in light of the history of troped liturgical texts. Next, we will account for the historical diversity of the troped ‘Kyrie’ genre while emphasizing importance of the Trinitarian elements. The third section recounts recent work on Aquinas’s theory of the passions and their effects of language formation; this leads to the fourth section, which casts a philosophical eye on the role of melisma in the liturgy. To conclude this study, we suggest the restoration of some troped ‘Kyrie’ texts in the Roman Rite.