Today's societal challenges require social innovative processes of co-creating knowledge. As a central actor in knowledge creation, universities can effectively contribute to transformative social innovation by instituting organisational and institutional change. The paper analyses how different strategies, policy mechanisms and incentive structures affect the long-term sustainability of social innovation initiatives at universities. Empirical data from interviews with top management, researchers, teachers and students at Lund University shows that the institutional logic for the change process has manifested itself in the triple helix innovation model, emphasising industry-university-government collaboration for strengthening national industrial competitiveness. This narrow interpretation of the third mission is challenged by internal as well as external pressures for the university to actively participate in solving social challenges beyond the sphere of industrial application. The case study illustrates the dynamics of competing institutional logics and how a traditional and research-oriented university struggles to embed social innovation ideas and practices into its policy framework.