Background: Citizen participation is integral to the governance of sustainability transformations. Long-term participatory processes undergo various phases of opening up and closing down various scopes of the participation—with significant consequences for the legitimacy and impact of the participation process. Methods: To gain a better understanding of these processes, we address the question of how and why participation processes are opened up or narrowed down. Through document analysis and key-informant interviews, we evaluate a case of long-term citizen participation linked to the development of a sustainable neighborhood energy system in northwestern Germany. Results: Results show that normative, substantive, and instrumental imperatives contribute to opening-up dynamics in participation processes. Closing-down dynamics were observed in the narrowing of thematic, spatial, temporal, and methodological scopes, as well as in the range of the actors involved. Reasons for closing down were financial and temporal restrictions, conflicting interests, the need for expert input in decision making about highly technological questions, the institutionalisation of participation, and stakeholder fatigue. Conclusions: This study provides a new framework for analysing citizen participation while highlighting the complexities and interrelations associated with citizen participation within the context of technological and urban development.