Idiomatic expressions are not just neutral equivalents of their literal counterparts, but exert some sort of evaluative force. This study employed the appraisal system (AS) to explore the functions of idiomatic expressions and their variations in English as a lingua franca context, and to compare different academic domains in terms of use of evaluative language in the form of idiomatic expressions. To this end, the corpus of English as a lingua franca in academic settings and the Vienna–Oxford International Corpus of English were searched for idiom tokens through whole-phrase and keyword search. The idioms and their variations were classified based on the AS. The findings indicated that social and behavioral scientific texts included higher proportions of evaluative idiomatic expressions whereas medical and natural scientific texts generated lower percentages of appraisal idioms. There was a stronger tendency in both corpora toward using idioms as appreciation as compared to judgment and affect. Given the employed methodology, the AS could not be applied to some idioms for their compositionality nature. © John Wiley and Sons Inc. All rights reserved.