Open innovation is an innovation framework proposing that established firms use external sources as pathways to new ideas, technologies, business models and markets. Within this framework, established companies can use startups, or young, growth-oriented business to help them achieve radical or breakthrough innovations. In this paper, we focus on established firms which use “corporate accelerators” to run fast-moving, competitive programs in which startup companies participate. Our purpose is to identify inhibitors to the collaboration between established firms and startups in these accelerator programs. We conducted 27 interviews with participants from startups, established companies using startups as innovators, and the accelerator management who provided the platform for this engagement. Our theoretical framework is the social realist theory of Margaret Archer. This provides a conceptualisation of the reflexivity of the participants and the “situational logic” of conflict and competition in which they find themselves. We found that collaboration will be inhibited by conflicts in basic beliefs, or propositions, about concepts such as authority, autonomy and risk, as well as competition for material resources and personal goals.