Since the 1990s, the European Union has sought to create a harmonised socio-economic classification scheme which symbolically unifies the social structures of the 28 member States in a common tool of description. Relying heavily on expert networks, this project, with numerous potential policy applications, has so far failed to come to fruition. After examining the scientific networks and institutional resources of an initial model which was at the centre of discussions for nearly ten years, the article explores the reason for its ultimate failure. Combining science studies and a political sociology approach, the article highlights the effects of conflicting legitimacies within expert networks, the lack of diversified institutional resources and contacts of the dominant experts, the poor fit of sociologically based methods to an EU administrative culture more focused on knowledge based on economic methodologies, and the lack of a wider European debate and mobilisation on inequalities which, in national histories, were key to the creation of national socio-economic classification schemes.