User acceptance models such as the technology acceptance model, the theory of reasoned action, and the theory of planned behavior have been widely used to study a specific information system, a group of systems, or even computers in general. This study examines the usage of competitive information systems. It applies the theory of planned behavior (TPB) in a comparative frame of reference model (relative model) in which relative attitude, relative subjective norm, relative intention, and relative usage are examined. The study is set in the context of two instant messaging technologies. Based on a survey from 300 instant messaging users, the effects of attitude and subjective norm on intention in each model were different (i.e., when TPB is tested once for each application). This confirms that the behavioral model can show different effects for competitive products. In addition, correct competitive answers were given by the relative model; however, these may differ from the answers found from a single application model. The authors show the importance of studying the relative model for competitive products.