Since H. Humbach's Baktrische Sprachdenkmäler (Wiesbaden, 1966) the main etymological proposal for Bactrian xpovo '(calendar) year, (regnal) year' has been A. Thierfelder's suggestion of a loanword from Hellenistic Greek xpovoς 'time'. In this article the plausibility of this etymology is re-examined, and it is further argued that it should be rejected on the grounds that the formal phonological differences between the potential Hellenistic Greek source form and its presumable loan-adaptation form in Bactrian are inconsistent with what is known of Bactrian diachronic phonology.