Intermediary actors have been recognized for their role in facilitating collaborations to achieve sustainability goals. Yet how voluntary sustainability collaborations, guided by intermediary actors, unfold in practice remains underdeveloped. Building on empirical data from the Principles for Responsible Management Education (PRME) network and using a network brokerage lens, we unfold dynamics of voluntary sustainability collaboration in the presence of an intermediary actor. We find that intermediaries act as sustained iungens brokers in voluntary sustainability collaborations, by connecting other actors and coordinating new collaborations in a multi-level network. Their activities are affected by tensions inherent to iungens brokerage in a voluntary setting, and factors influencing the success of the network collaborations. We contribute to the sustainability literature by offering a firsthand view on the dynamics of sustainability collaborations through an autoethnographic approach. We provide implications for sustainability collaborations in voluntary settings and recommendations for advancing responsible management education through collaboration.