Prior research in the context of PPPs has mainly focused on organizational and risk management aspects. While there is a growing number of IT megaprojects in the context of PPPs, we still have a lack of understanding on the role of boundary spanning between public and private parties for successfully bridging expectation gaps among the partners during the realization of IT-PPP-megaprojects. In this exploratory, qualitative single-case study from the German TollCollect IT megaproject we draw on findings from boundary spanning literature to explain boundary spanning activities contributing to trust formation in IT-PPP-megaprojects. Our developed grounded model suggests that the deployment of boundary spanning activities and their specific antecedent conditions, moderated by external stakeholder support of a public-private environment, affects the formation of mutual trust and therefore the success of an IT megaproject in the context of PPPs.