Introduction: The ultimate viability of an information system is dependent on individuals' continued use of the information system. In this study, we use the technology acceptance model and the theory of interpersonal behaviour to predict continued use of information systems. Method: We established a Web questionnaire on the mySurvey Website and recruited participants via social networks. Data were collected from a sample of 1,154 Webmail users in Taiwan. Analysis: Our models are analysed using the partial least squares structural equation modelling method. Results: The results of this study indicate that (1) the theory of interpersonal behaviour has better ability to predict continued use than the technology acceptance model; (2) the technology acceptance model has a better ability to predict intention to continue use than the theory of interpersonal behaviour; (3) the combination between the technology acceptance model and the theory of interpersonal behaviour is properly able to explain continued use of information systems; and (4) multiple group analysis indicates that two relations in the combined model differ significantly between light experience users and heavy experience users. Conclusions: The technology acceptance model and the theory of interpersonal behaviour should not be viewed as alternative but as supplementary models. Moreover, the integration of such two models can properly predict continued use of information systems.