This study investigates whether conceptualizable agents in the discourse play a role inEnglish L2 overpassivization errors (nontargetlike passivization of unaccusative verbs, a subclassof intransitives). It hypothesizes that learners are more likely to make overpassivization errors inexternally caused events (in which an agent or cause may form part of the speaker's mentalrepresentation) than in internally caused events (in which the cause or causer of the event is notclear). Advanced Chinese learners of English were asked to choose the more grammatical form(active or passive) in target sentences with unaccusative verbs. Each target sentence wasembedded in two different contexts expressing external and internal causation. A significantdifference in error rates was found between the two different contexts: Learners acceptedpassivized unaccusative verbs more frequently when an agent or cause was available than whenit was not. This finding is taken as an indication that learners transitivize unaccusative verbsbefore they passivize them and that the degree of transitivization varies depending on thepresence of conceptualizable agents in the discourse. Thus this paper argues against a purelysyntactic analysis of interlanguage errors such as overpassivization and in favor of an approachthat takes cognitive factors into account.