The purpose of this investigation was to use Rasch measurement to study the psychometric properties of a 34 item questionnaire designed to measure second language learners' willingness to communicate (WTC) in English inside their language class. 490 Japanese university students' responses to the questionnaire were subjected to a number of different analyses. The first involved a comparison of the category threshold estimates produced by the Rating Scale and Partial Credit models. The questionnaire's items were then evaluated according to how well they defined the willingness to communicate construct. The potential dimensionality of using items that involved different speaking and writing tasks/situations in order to gain a more comprehensive understanding of students' willingness to communicate was also investigated. Next there was an examination of the questionnaire's four-point scale to ensure that it captured meaningful differences in students' WTC. Finally, the questionnaire items were compared using differential item functioning to determine if second year students were more willing than first year students in any of the different speaking and writing tasks/situations. This investigation closes with some suggestions on how the WTC questionnaire can inform second language instruction and curriculum design.