Scientists working on ecosystem service (ES) science are engaged in a mission-driven discipline. They can contribute to science-policy interfaces where knowledge is co-produced and used. How scientists engage with the governance arena to mobilise their knowledge remains a matter of personal choice, influenced by individual values. ES science cannot be considered neutral and a discussion of the values that shape it forms an important part of the sustainability dialogue. We propose a simple decision tree to help ES scientists identify their role and the purpose of the knowledge they produce. We characterise six idealised scientific postures spanning possible roles at the science-policy interface (pure scientist, science arbiter—guarantor, issue advocate—guardian, officer, honest broker and stealth issue advocate) and illustrate them with feedbacks from interviews. We encourage ES scientists to conduct a reflexive exploration of their attitudes regarding knowledge production and use, with the intention of progressing toward a higher recognition of the political and ethical importance of ES assessments.