The present paper is devoted to the understudied corpus of Byzantine Euchologia preserved at St Catherine’s Monastery (Sinai). It focuses on those copied in Greek language and offers a first overview on their scribes and users, especially thanks to an analysis of the annotations in Greek and other languages (Arabic, Armenian, Coptic) which they often carry. Who had access to Greek prayer books? Which kind of interventions do we come across? Used by people of different regional origins and linguistic traditions, these books enshrine a real ‘stratification’ of languages, practices, and traditions. A descriptive list of the 30  Euchologia manuscripts ‘stricto sensu’ written before 1453 is given at the end of the contribution.