This study investigates the effects of idiom transparency and context on Japanese EFL learners' interpretations of unfamiliar English idioms. Eighteen second-year Japanese university students were presented with 24 unfamiliar English idioms, some with and some without supportive context. Participants performed an explanation task in which they wrote down what they thought each idiom meant, and a forced choice task in which they selected one of four possible definitions for each idiom. Results for both tasks showed that participants were more successful interpreting high-transparency idioms (e.g. hang by a thread) than low-transparency idioms (e.g. cool one's heels), and more successful interpreting idioms presented in supportive context than those in non-supportive context. However, there was no interaction between transparency and context. The results are discussed with respect to models of L2 idiom interpretation.