In the context of the research programme ‘Idiomatic Second Language Acquisition’ (for more information see http://isla.ruhosting.nl), we collected normative data of 374 Dutch idiomatic expressions by 390 native speakers. In an online test, we asked participants to judge various dimensions of idiomatic expressions on a five-point scale: Frequency, Usage, Familiarity, Imageability, and Transparency. In addition, we objectively assessed their knowledge of idiom meaning by means of a multiple choice question (Idiom knowledge recognition). The dataset contains the aggregated results per expression for the 5 subjective dimensions (Frequency, Usage, Familiarity, Imageability, and Transparency) and the objective Idiom knowledge recognition (proportion correct).This work is part of the research program Free Competition in the Humanities with project number 23000349 NWO ISLA FdL, which is financed by the Netherlands Organisation for Scientific Research (NWO).