Mobility as a Service (MaaS) involves integration of various forms of public and private transport services into a single mobility service, accessible on demand. For MaaS to become successful, different suppliers of transport services have to cooperate in alliances in order to bring new benefits in the short and long term. Past experience demonstrates that this is a challenge, and existing transport providers are struggling with this cooperation. Various factors, including divergent interests of stakeholders, may limit the formation of such alliances. In this paper, we resort to the theories of alliance formation to extend our understanding of the formation of alliances within MaaS. Based on the economic, sociological and business literature we propose a conceptual model and formulate ten fundamental propositions for alliance formation to offer MaaS systems. The model takes the perspective of business firms for whom the institutional environment is an exogenous influence. We then apply this conceptual alliance-formation-model for a MaaS-pilot in Nijmegen, The Netherlands. Stakeholders within this pilot were interviewed on their conditions for forming an alliance. It appears that shared goals, limited risks for the partners involved, trust, and stimulating public actions are crucial for a successful alliance. For the pilot, however, learning appears to be the main motive for the firms to get involved. For future transition from the exploration to the exploitation phase of MaaS alliance, these results should be taken into account. The framework and the propositions developed in this paper could be adopted as the necessary preconditions for designing a proper governance structure for providing MaaS services.