Unconferencing is a method for organizing social learning which could be suitable to trigger sustainability learning processes. An unconference is defined as participant-driven meeting that tries to avoid one or more aspects of a conventional conference, such as top-down organization, one-way communication and power-relationships based on titles, formal hierarchies and status. This paper presents a case study on the application of unconferencing in a large Swiss university (ETH Zurich) where an unconference was conducted to engage students, academics, staff and external experts in a mutual learning process aimed at the development of project ideas for reducing its CO2 emissions. The study analyzes how the unconferencing format initiated and promoted sustainability oriented group processes during the unconference, and in how far the projects which were developed contributed to a reduction of the university’s CO2 emissions.