Information systems researchers have applied the theories of reasoned action (Fishbein and Ajzen 1975) and planned behavior (Ajzen 1985, 1988) to predict user intention and usage of information technology (IT). Most studies have assumed independent relationships among the determinants of user intention and usage. It is important to examine the interdependencies among these variables to avoid model misspecifications and misinterpretation of results. Using a longitudinal sample of 136 users, this study tests interdependencies among the determinants in the theory of planned behavior model of IT usage. The results generally support the predictive validity of the theory and its hypothesized paths with the exception of a nonsignificant link from subjective norms to behavioral intentions. Six hypotheses concerning the crossover effects between the antecedent variables were supported. The addition of these crossover paths significantly improved the overall model fit. These findings suggest that the structure underlying the theory of planned behavior is richer in content and more complex than is often presumed, particularly with regard to the normative components.