This paper critically appraises the idea and practice of ‘participation’ in scientific environmental research, arguing for the wider uptake by physical geographers of a more radical participatory approach. It proposes participatory action research (PAR), which offers an alternative mode of science, involving collaboration and co-production of research from question definition through to outcomes. We begin with a critical view of public participation in environmental research and policy-making to date. We argue that much rhetoric and practice of participation is shallow, focusing simply on including relevant publics and stakeholders, or having an underlying agenda of building trust in science or policy-making. Both orientations diverge drastically from the radical traditions in which participatory research and planning originate. In the rest of the paper, we illustrate an alternative process of knowledge co-production, reporting on a PAR project on farm slurry pollution conducted with a UK Rivers Trust. We evaluate the knowledge co-produced, the responses of participants and the scientific process. Suggesting that we reframe co-production as the circulation of expertise, we argue that PAR can enrich the learning, knowledge and skills of all those involved and lead to innovation and positive environmental outcomes. A number of structural and institutional barriers to deep participatory processes need to be addressed.