Lexical inferencing is considered a listening strategy that is commonly employed by advanced EFL (English as a Foreign Language) listeners and a factor that contributes to successful listening comprehension. However, investigations of the factors that influence inferencing success in listening as well as how much each factor contributes to success are scant, as more studies have been conducted exploring lexical inferencing in reading. In addition, even though idiomatic expressions such as smell a rat, jump the gun, and go cold turkey are ubiquitous in the English language, especially in oral communication, and they are considered crucial in both first language (L1) and second language (L2) acquisition, little is known about the effectiveness of inferencing strategies where idiomatic expressions are concerned.Three goals motivated the current study. The first goal was to investigate whether inferencing is an effective strategy in the case where the target item is an idiomatic expression. The second goal was to investigate how four person-level factors, familiarity, listening proficiency, listening vocabulary size and working memory, two sentence-level factors, lexical density and sentence length, and two lexical-level factors, L1–L2 congruency and semantic transparency, influence the inferencing success of English idiomatic expressions in listening. The third goal, related to the second goal, was to determine which of the two lexical component factors, L1–L2 congruency and semantic transparency, is more important to inferencing success. A mixed methods design, the explanatory sequential design (Creswell & Plano Clark, 2018), was employed in this study. Quantitative data were collected from 89 EFL Japanese university students using a Listening Vocabulary Levels Test, a Listening Span Test, and an Idiom Inferencing Elicitation Task. The collected data were examined using mixed-effects logistic regression. Twelve participants were invited to participate in follow-up interviews based on their response patterns on the Idiom Inferencing Elicitation Task. The quantitative results indicated that familiarity, listening comprehension skills, working memory, and L1–L2 congruency were significant factors influencing inferencing success and the qualitative results supported these findings. In addition, the qualitative analyses suggested that depth of vocabulary is another potentially important factor. Furthermore, listening comprehension moderated the L1–L2 congruency effect. The finding that semantic transparency is not an influential factor in successful inferencing of unfamiliar idiomatic expressions provides evidence that the semantic transparency of known idiomatic expressions formed after learners acquire the meaning of the expression is a different construct from the perceived semantic transparency of unfamiliar idiomatic expressions. In addition, even though the sentence-level factors were not statistically significant in successful idiom inferencing in this study, further studies are required in order to see if this result holds true when the characteristics of the listening tasks differ from those of the task used in this study. It is hoped that the findings provide insights into how to help Japanese university EFL learners improve their listening skills, especially in tasks that include unfamiliar idiomatic expressions. / Teaching & Learning