This study investigates the second language (L2) acquisition of the English count/mass distinction by speakers of Korean and Mandarin Chinese, with a focus on the semantics of atomicity. It is hypothesized that L1-Korean and L1-Mandarin L2-English learners are influenced by atomicity in the use of the count/mass morphosyntax in English. This hypothesis is tested in two experiments, one comparing Korean and Mandarin speakers in their L2 (English) and the other investigating count/mass morphosyntax in native Korean and Mandarin Chinese. In both experiments, participants are tested on their suppliance of plural marking with count and mass NPs. The findings are fully consistent with the view of atomicity as a semantic universal: learners overuse plural marking with mass atomic nouns such as "furniture" more than with mass non-atomic nouns such as "water." Even though plural marking is associated with atomicity in Korean but not in Mandarin, the same patterns are observed in L1-Korean and L1-Mandarin L2-English learners. We conclude that learners' performance is not due to L1-transfer, but rather to the role of the semantic universal of atomicity in L2-acquisition.