The present study investigated the conventional vs. nonconventional use of idioms in general and academic English as a lingua franca (ELF) corpora taking into account the speech event type, academic domain, and discipline. ELFA and VOICE corpora were searched for idiom tokens based on Collins COBUILD Idioms Dictionary. Results showed that idioms were more frequent in VOICE than in ELFA, indicating a higher proportion of formulaic language in informal and interactive discourse as compared to more formal and transactional discourses. Tokens in conventional form and meaning were the most frequent in both corpora. Entirely novel idioms were small in number in both corpora. However, both corpora generated a large number of idioms with formal variations. Idiom use in the academic corpus was register sensitive. ELF speakers in both corpora used communication strategies to prevent unilateral idiomaticity. Overuse of high-frequency idioms by some speakers could be associated with idiomatic teddy bears. Results can help understand the nature of idiomaticity in ELF in general and academic settings. Findings on the academic corpus can also inform curriculum development and assessment in English for Academic Purposes. © 2022 - Published by Shahid Chamran University of Ahvaz. All Rights Reserved.