The corpus of texts transmitted under Ptolemy's name includes works surviving in whole or part in Greek as well as works or parts of works that exist only in Arabic or Latin, ostensibly translations of lost Greek originals. The authenticity of some of these texts, for example the Almagest, is unquestioned, whereas others are without doubt spurious. Recent opinions by competent scholars have been divided on the authenticity of two works, On the Criterion and the Governing Faculty, which is extant in Greek, and Optics, which we have only in a Latin translation of a lost Arabic translation. In the present paper I describe certain stylistic elements shared by the accepted Greek works and by the Criterion, which are either not found at all in other ancient Greek authors or only in a very few later authors who are demonstrably influenced by Ptolemy. As well as providing a secure standard for establishing common authorship, these enhance our general understanding of Ptolemy's style (as previously investigated by Boll and Stückelberger among others). I conclude with a consideration of the question whether stylistic evidence for authorship can be extracted from the works that are not extant in Greek.