How engineering in the context of urban socio-environmental challenges is practically and effectively mobilized has been the subject of some debate. Numerous professional bodies have encouraged engineers to approach socio-environmental issues through increased engagement with, and accountability to, the public through effective participatory practices. This article presents a close empirical analysis of a major engineering project in London to argue that engineering has a more complex relationship with social, political and environmental conditions than the idealistic participatory conception supposes. In fact, the spatial, technical and economic arrangements of engineering practice may limit the potential for public participation. Through a detailed analysis of the example of the London Water Ring Main (from around 1988 to 1994), this article shows how myriad sometimes conflicting engineering issues and responsibilities interfered with key elements of effective participation. Therefore, although increased public engagement in engineering may be desirable in theory, substantial professional, institutional and political change may have to occur before this is possible in practice.