Previous studies of washback (the influence of a test on teaching and learning) have provided insights into the complexity of educational systems and test use, especially in relation to the role of the teacher, but have given insufficient attention to the relationship between observed practices and test design features. In this article a washback model is proposed that incorporates both test design and participant characteristics. The model is used to predict behaviour on preparation courses directed toward the Academic Writing component of the International English Language Testing System (IELTS) test. 197 learners and 20 teachers were observed over 51 classroom hours. These encompassed 22 IELTS preparation classes and, for comparison, 13 classes of English for academic purposes (EAP). Evidence was found for substantial areas of common practice between IELTS and other forms of EAP but also for some narrowing of focus in IELTS preparation classes that could be traced to test design features. (Contains 9 tables and 1 figure.)