Purpose: Advanced skills in communication, teamwork and stakeholder engagement are widely recognized as important success factors for advancing sustainability. While project-based learning formats claim to advance such skills, there is little empirical evidence that demonstrates how interpersonal competence is being developed. This study aims to describe and explains teaching and learning processes of project-based sustainability courses that contribute to the development of interpersonal competence as one of the key competencies in sustainability. Design/methodology/approach: This study on an international project-based learning course adopted a multi-method case study approach, triangulating observations, semi-structured interviews and focus groups supported by Photovoice method through which students tracked their learning processes. Data collection and analysis followed a grounded theory approach. Findings: Learning through and from conflicts within a learning community can foster competence development in teamwork, communication and stakeholder engagement. This study identified inner and outer conflicts (within individuals versus between individuals or groups) as potential drivers of learning processes, depending on strategies applied to address these conflicts. Originality value: The value of this study is fourfold: it demonstrates how conflicts can be leveraged for students' competence development; it provides in-depth empirical data from multiple perspectives, it discusses the findings in the context of teaching and learning theories, and it demonstrates an application of the Photovoice method to track and improve teaching and learning processes.