This paper investigates performative manifestations of sincerity across Anglo-Norman and Middle English. In particular, it locates adverbial sincerity markers used to qualify performative speech act verbs in late medieval letters (fourteenth and fifteenth centuries), at a point when Middle English was rapidly replacing Anglo-Norman as the vernacular of epistolarity in England. Employing historical dictionaries and corpora, the study (1) locates the range of words for 'sincerity' from a time when the modern lexeme had yet to be borrowed in either vernacular, and (2) demonstrates that while it is clear that Middle English epistolarity was greatly influenced by Anglo-Norman, quantitative and qualitative analyses suggest that sincerity markers were much less commonplace in Middle English performatives, which further suggests ways in which the communicative ideal and practice of sincerity were reanalyzed from one language to the next.