There has been increased interest in examining the relationship between different linguistic modules in second language learners' grammar system. One such interface concerns learners' ability to map morphosyntax to target-like semantic interpretations, especially in cases where the first and second languages differ in morphosyntax-semantics mapping. We examined this issue by investigating Chinese-English bilingual adolescents' knowledge regarding the semantic consequences of mass-count morphosyntax in English. 228 Chinese-English bilingual students from Singapore took part in a quantity judgment task. Following previous studies, we tested five noun conditions including count (e.g., shoe), substance mass (e.g., ketchup), object mass (e.g., furniture) and two conditions involving items that can occur flexibly in both mass and count contexts (e.g., string/strings). The last two conditions specifically probe learners' ability to make use of morphosyntax in deriving semantics. The representation of quantity for objects/substance was manipulated for number or combined volume (e.g., two large shoes/portions of ketchup versus six tiny shoes/portions of ketchup). Participants were asked to make judgments on the quantity of items, in response to questions presented in a count and/or mass frame (Who has more shoes/ketchup/furniture/string/strings?). Results show that our bilingual participants were able to make appropriate semantic judgments of quantity in response to manipulation of morphosyntax. We compare our findings with previous research in first and second language contexts and discuss the differences in terms of the quantity and quality of input in shaping learners' grammar system.