<span>Citizen science can expand data collection and connect research with society, but involving non‑scientific stakeholders introduces complex relations that challenge conventional geoscientific practice. This paper examines these dynamics through the lens of ISOSCAN, an interdisciplinary project that co‑designed a citizen science programme for collecting stable water isotope samples across Scandinavia. Co-design workshops, prototype testing and the analysis of five <em>vignettes</em> (short narratives) authored by scientists were used to uncover these complex relations. Three themes emerged: epistemic tensions between scientific precision and diverse stakeholder objectives; mutual learning that reshapes disciplinary assumptions and practical protocols; and the centrality of trust and iterative processes over strict methodological control. We make sense of the complex relations and offer insights into how co-designed citizen science can promote interdisciplinarity and methodological innovation in geoscience.</span>