Many people have experienced problems and frustrations with unexpected responses when operating computers or appliances. An objective index of such problems was developed using brain potential responses. Event-related potentials from expected and unexpected visual stimuli that were presented after voluntary actions were recorded and compared, in order to identify the specifi c components that refl ect the cognitive mismatch between action-based expectations and outcomes. Results indicated that a late positive potential (LPP) in a latency range of 500–700 ms was elicited to a cognitive mismatch, regardless of the choice of action, task instructions about the occurrence, or the type of expectation. Moreover, the amplitude of the potential decreased after several repetitions of the cognitive mismatch. Based on these results, a model of brain potential responses to cognitive mismatch is suggested. Furthermore, a practical application (i.e. usability testing) of the LPP was indicated. This study has increased the basic knowledge about objectively measuring of cognitive mismatch.