The present study aimed at identifying and quantifying the idioms used in three ILI "Advanced" level textbooks based on three different English corpora; MICASE, BNC and the Brown Corpus, and comparing the frequencies of the idioms across the three corpora. The first step of the study involved searching the books to find multi-word idiomatic expressions used in each. Idioms matching criteria for idiomaticity were selected and searched in the three online corpora to find their frequency of occurrence. Chi-square tests were then run to discover whether there were significant differences among the frequencies of occurrence of each idiom across each corpus. Having the number of idioms in each textbook, two other chi-square tests were then run, the first aiming at finding out if there were any significant differences among the three books in terms of idiom types and the second, to compare their tokens. The results showed that the books were different in terms of both number and type of idioms. It was also found that the idioms chosen for these "Advanced" level books did not meet necessary frequency criteria according to the literature, which could be attributed to representativeness issues of the corpora or their scope in terms of language level, genre and speaker's age.