The benefits of Internet-based health promotion programmes are much discussed, yet the literature on their feasibility and utility is limited. Here, we evaluate an Internet-based exercise motivation and action support system (Test system), relative to a group receiving no intervention (Reference) and another receiving a less interactive version of the same system (Control). We report results from a 10-week pilot study with 75 participants aged 23–54 years. We found that, relative to the control system and reference group, the more interactive (test) system was more engaging (better user retention), created higher expectations for exercise, greater satisfaction with motivation and increased self-perception of fitness. Seven months after the intervention, participants who used the test system reported greater levels of increase in exercise than the control or reference groups. Our study adds to the growing body of evidence on the benefits of interactive systems and the role they could play in health promotion programmes. However, it should also be noted that not all Web-based systems offer the same level of advantage; careful design is crucial to ensure that key messages are clearly communicated to attentive users. Further research is required to identify the optimum interactive system design principles, across a range of user types.