Purpose –The purpose of this study was to empirically examine some common aspects of knowledge and project management. Using the contingency theoretical approach and addressing the current lack of empirical evidence on the issue, the study aimed to investigate the potential mediating role of project complexity in the impact of different types of knowledge assets on project success. The research was carried out in the context of information systems (IS) projects. Design/methodology/approach –The field survey was adopted as the most appropriate research method for this study because of its versatility, efficiency and generalisability. Usable data were obtained from 603 IS professionals across a variety of projects and were analysed through descriptive statistics. The collected data included participants’ perceptions of: (1) project success as the ultimate target; (2) project team, customer and process representing human, relational and structural capital factors of the project success; and (3) project complexity as a potential mediating factor. Originality/value –The results of this study make two important contributions to research: (i) they fill the lack of empirical evidence; and (ii) provide valuable insights on how different project characteristics and knowledge assets influence project success. Specifically, external relational capital (project customer) was found to be more important than internal human and structural capital (project team and process) in moderately to highly complex projects. For simple projects, structural capital (project process) was more important than the other two types of knowledge assets (project team and customer). Such findings are consistent with the contingency theory propositions in knowledge management. Practical implications – The findings of the study provide useful guidelines for project managers on how to best organise and utilise their available knowledge assets in alignment with project complexity in order to enhance project success. Nevertheless, these implications need to be interpreted with caution due to the limitations of the survey method and its application in a specific context (IS projects) and among specific subjects (IS professionals). Future research is recommended to address these limitations and extend current research to include both static (knowledge capital) and dynamic (knowledge practice) aspects of knowledge management in project management.