While there is now a large amount of social science research on scientific expertise, testimonies made by sociologists who themselves participated in scientific expertise on a controversial topic remain rare. It is this type of feedback and testimony that this paper will articulate and discuss. The aim is to propose a series of reflections on scientific expertise from a personal experience: the participation of the author as a sociologist in an expert committee set up by the former French Agency for the Safety of Health, the Environment and Work (AFSSET) on the topic of radio-frequencies. Several problematic aspects of scientific expertise will thus be discussed from this concrete experience: the problem of the composition of the expert group and the issue of conflict of interest, the way in which the work of expertise is organized within the group, the effects of the presence of an observer from an association, and the differences between scientific work and scientific expertise.