The current study examined the second language (L2) acquisition of a specific case of one-to-many mapping, the adjective different and its two partially synonymous Japanese translations, betubetu and tigau. This particular example was recognised by Duffield, Matsuo and Wood (2005) who reported findings from Japanese native-speakers indicating that the word betubetu is more restricted in meaning than both different and tigau, and that English L2 learners were likely to overextend its use. Duffield et al.’s results had broad scope for interpretation and the present study investigated their significance within a framework of first language (L1) to L2 semantic transfer. Two experiments were conducted to explain English L2 learners’ performance at a lexically-specific level as well as to inform on the processes generally underlying L2 semantic development. A novel-word learning task and a magnitude estimation study of semantic acceptability used cross-linguistic comparisons of English and German learners to identify L1 dependencies in L2 behaviour. Methodological difficulties limited the explanatory potential of the novel-word task but magnitude estimation showed significant differences between native-speaker and L2 learner acceptability judgements consistent with the involvement of L1 semantic preferences in L2 lexical processing. As numbers in the study were small and learners varied in their L2 experience, the extent to which these deviations directly reflected L1 semantic influences could not be firmly concluded. However, their implications for current approaches to semantic development are discussed.