The concept of sincerity has links to honesty, openness, and authenticity, including of feelings. As expressions of sincerity become formalized in epistolary practice, however, a tension arises between sincerity and the articulation of it. An examination of a corpus of private family letters in French and Spanish from the eighteenth and nineteenth centuries shows that use of the word ‘sincerity’ is much more common as an epistolary formula in French, where it had a broader semantic range, while authors writing in Spanish use other methods to index truth and emotional openness. The most frequent users of this formula are shown to be less skilled writers and bilinguals writing in their second language, suggesting a greater reliance on preconstructed formulae, especially in situations of increased linguistic and social distance. Finally, some bilingual authors transfer the pattern into Spanish, indicating that fixed phrases and formulae are available in a bilingual’s linguistic repertoire for pragmatic redeployment.