This paper considers evidence for diachronic change in medieval mixed-language business writing produced in London in the fourteenth and fifteenth centuries. The relevant languages were Middle English, Anglo-Norman and Medieval Latin, and the mixed-language system is briefly described, consisting of function words in either Medieval Latin or Anglo-Norman (depending on which was chosen as the matrix), and an amount of lexical items in English. The changes discussed concentrate around the end of the fourteenth century, which is when English resurfaced as a written language (the Norman administration had replaced English with Latin and French for written purposes). Further, it seems likely that spoken Anglo-Norman, as a language of bilinguals in Britain, died out at the end of the fourteenth century, which seems to have been pivotal.