Although individual adoption and use of a single system has been examined extensively, little is known about how people evaluate and compare competing systems. In this paper, we discuss and test three alternative models underlying user comparison of competing systems: separate, crossover effect, and relative comparison processes. The separate comparison process proposes that users develop separate cognitive, affective, and conative evaluations toward each system, and the between-system comparison only occurs at the point of choosing a preferred system. The crossover effect comparison process posits that users not only perform separate evaluations for each system, but also consider the competitive effects when proceeding across cognitive, affective, and conative evaluation stages. In contrast, the relative comparison process postulates that users directly compare competing systems within each of the cognitive, affective, and conative evaluation stages. Based on the IS continuance model, we tested each of these three models using data collected from users of two competing instant messaging systems. Our results showed that the relative comparison process is the most parsimonious and the best model in terms of explaining the mechanisms underlying the comparison of system use by individuals. Theoretical and practical implications are discussed.