Purpose: This paper aims to connect participatory sustainability implementation with sustainability assessment, exploring learning theories, the principles of Higher Education for Sustainable Development (HESD) and respective indicators applied in the university context. Even though participation is partly considered in existing assessment practices, it is still unclear what and how to measure participatory processes that envision implementing sustainability principles in higher education institutions. Holistic approaches are often proclaimed, but reductionist assessment methods are frequently followed. Design/methodology/approach: The study followed a qualitative approach, inspired by the Delphi method, and includes semi-structured expert interviews (N = 15) and two focus group discussions (N = 23), with participants coming from a total of 17 different countries. Data were analysed and compared according to qualitative content analysis and systemized according to the underlying theoretical strands. Findings: The findings suggest that participatory processes can be better assessed from a social learning and organisational learning perspective, emphasizing non-linear criteria for the quality of the process in terms of depth and meaningfulness as well as criteria for the quality of the outcome in terms of knowledge generation and innovation. The findings also point implicitly to the need of considering double- and triple-loop learning, if a culture of participation towards sustainability is to be pursued, and underline the high impact of institutional governance. Originality/value: Although a great volume of literature about sustainability implementation in higher education exists, studies focusing on participatory processes in this context are rather scarce. This research pays attention to sustainability experts working in universities rarely heard in a more systemic manner and also applies a reflective participatory approach itself by using qualitative methods.