Mutual learning is a fundamental element in transdisciplinary (Td) sustainability research. It allows for integrating knowledge and experiences gained in different contexts, including the building of consensus about necessary transformations to reach sustainability solutions. To successfully achieve mutual learning, appropriate conditions are required. These range from providing boundary objects that serve knowledge integration, the development of a common language and knowledge, and shared experiences to transparency concerning the objectives and motives of all those involved. Mutual learning is particularly challenging from an intercultural perspective in both local and global processes. Interaction among the participants is based on experiential, educational, and cultural dimensions that induce different types of knowledge and cognition, thought styles, socializations, constraints and preferences in socio-political structures and day-to-day practices. In this article, formats of case-based Mutual Learning Sessions (cbMLS) that organize mutual learning based on a single case or a set of cases are presented. The different formats have been developed and studied in the context of a global Td process on phosphorus management (i.e., the Global TraPs project). After presenting the formats of cbMLS, the paper presents the first results from an empirical, integrated qualitative study based on interviews and observations, and discusses potentials and limitations of cbMLS, in particular their application in an international context. We further highlight challenges and learning experiences that have to be met in Td mutual learning processes.