A multidimensional item response theory model that accounts for response style factors is presented. The model, a multidimensional extension of Bock's nominal response model, is shown to allow for the study and control of response style effects in ordered rating scale data so as to reduce bias in measurement of the intended trait. In the current application, the model is also used to investigate response style as an underlying cause of differential item functioning. The approach is illustrated using the item responses of cigarette smokers to the Wisconsin Inventory of Smoking Dependence Motives, a self-report measure of tobacco dependence. (Contains 3 figures and 5 tables.)