There is a broad scientific consensus that our current mobility system, characterized by automobility and the use of fossil fuels, is unsustainable. In cities around the world, sustainable mobility initiatives are experimented with and implemented; especially is zero-emission bus technology a prevalent initiative. The socio-technical transitions view and its much-used frameworks, the multi-level perspective and transition pathways, has proven useful in investigating national, bottom-up sustainability initiatives. However, it pays little attention to space and scale. Other frameworks are better fit for investigating sustainability experiments in urban contexts. Therefore, I will use the theory of sustainability experiments and institutional theory to grasp the aspects of a transition not taken into account by the most prominent socio-technical approaches. Based on in-depth interviews with actors in Oslo`s public bus system, qualitative content analysis and observation, this thesis will provide new understandings of how a sustainable mobility initiative evolves in an urban context. I investigate the rationale behind the zero-emission bus experiments, who the initiative emerged, and what institutional barriers and enabling factors the experiments are exposed to. These barriers and enabling factors are important to map out before a potential full-scale implementation, as it is assumable that the barriers will remain in a full-scale implementation, if not lowered now. I have found that the greatest barriers associated with the zero-emission bus experiments do not derive from technical aspects. The associated charging infrastructure, a new element in Oslo`s bus system, function as a barrier in many domains. Despite many barriers, I have found several enabling factors for ZEBs. Also, the actors of Oslo`s bus system are showing contours of altering institutions that are important if a zero-emission bus implementation will be successful. By organizing zero-emission experiments, the novel technology can be protected from the normal selection environment, it accumulates important ZEB knowledge and it allows for a gradual up-scaling of technology. Based on my, and previous findings, I have crafted a tripartite conceptual framework of important institutional innovation activities that are important for a full-scale implementation of ZEBs.