The diffusion of sustainability as a social innovation at universities from the perspective of student initiatives. The successful implementation of sustainability at universities not only depends on sustainability governance structures, but also on new social practices. To initiate these social innovations as bottom-up processes, student initiatives are important actors. In this article, we applied Roger’s theory on diffusion of innovations to analyse such examples of diffusion processes at universities. Questions aim at the starting point, relevant communication channels, effects of homophily and the achievement of a critical mass. To investigate these, eight experts from six student initiatives were interviewed. A major challenge in the diffusion process turned out to be the establishment of constant interpersonal communication, especially with people beyond the student body. Furthermore, we cannot precisely identify a point of critical mass. Based on the key findings, we worked out practical recommendations for student initiatives. In conclusion, the analysis suggests that Roger’s theory is helpful to reconstruct diffusion processes, but only provides limited benefit to the direct support of current diffusion processes.