I examine a hypothesis concerning the decline of the past‐oriented aorist infinitive in Greek in a corpus of private letters dating to between the third century BC and the sixth century AD. According to the hypothesis examined, the decline of the past‐oriented aorist infinitive was related to the aspectual value of the aorist infinitive in Greek and to the spread of the perfect in the cross‐linguistic sense of the word (Haspelmath 1992). In addition to supporting the hypothesis examined, the corpus also sheds new light on the syntax of infinitive clauses in the period investigated. It is observed that, when found in the corpus, past‐oriented aorist infinitives tend to be used in specific, predictable contexts that support their past‐orientedness, and they have a parallel in the use of the aorist injunctive in Vedic Sanskrit. This phenomenon is consistent with the hypothesis examined in that it assumes that the aorist infinitive encoded the perfective aspect rather than tense.