This study adopted Ambridge's research paradigm to examine the effects of entrenchment, preemption, and verb semantics in second language (L2) acquisition of English "un-" prefixation. Three groups of Chinese learners of English (second- and fourth-year English majors and teachers of English) rated the acceptability of 48 "un-" prefixed verbs and their bare forms. Frequency of bare forms (e.g., "close") was a factor associated with the preference of the fourth-year learners and the teachers to reject ungrammatical "un-" verbs (e.g., "unclose"). However, competing forms (e.g., "open") did not appear to be linked to participants' judgments of the acceptability of ungrammatical "un-" verbs, lending support to the entrenchment rather than the preemption hypothesis. Ratings of the extent to which verbs incorporate the semantics of "un-" verbs emerged as significant predictors for the fourth-year learners' and the teachers' judgements of the acceptability of grammatical "un-" verbs, supporting the semantic verb class hypothesis.