The present study addresses Korean EFL learners’ difficulties in preposition learning due to cross-linguistic differences between Korean and English at the lexico-semantic level. Specifically, it attempts to explore Korean learners’ productive knowledge of English prepositions that are not lexically differentiated in Korean (overdifferentiated prepositions (ODPs), henceforth) such as until and by. A total of 109 adult Korean learners of English and seven native English speakers participated in the study and were asked to provide prepositions appropriate to the context in a cloze test. The analysis of their responses revealed that Korean learners indeed make production errors due to their confusion between prepositions in ODP pairs. Learners tended to show biased use of ODPs, overusing one member in an ODP pair over the other. While the accuracy of using ODPs in their proper context increased as the learners’ proficiency increased, even advanced learners recorded low accuracy in differentiating some ODPs such as on and above. The findings highlight the need for special attention to L1-L2 differences at the lexico-semantic level when teaching prepositions.