This paper is about the peculiar particularities of the dual trends towards urban water privatization and commodification. It uses as its analytical entry point the extraordinary emergence of large-scale seawater desalination, delivered through public-private partnerships, as an alternative municipal water supply for the San Diego–Tijuana metropolitan region. The paper engages and extends Karen Bakker’s work on water as an ‘uncooperative commodity’. Interrogating the neoliberalization of water through desalination, it is argued, requires reference to the socio-technical relations drawn together under the ‘desalination assemblage’. Such water treatment technologies –and the social relations that flow through them– are, in other words, efficacious in the market-disciplining of water. The paper presents an understanding of privatization and commodification as diffuse, and as unfolding through multiple and contradictory materially heterogeneous relationships. Drawing on both urban political ecology (UPE) and assemblage thinking, the paper calls for a more constructive dialogue between different concepts of socio-material relationality. The empirical case studies of two large seawater desalination plants (one in Southern California, one in Baja California) and the re-configuring relations of public/private water governance associated with these projects, provides a pertinent imperative for greater attention to be paid to contingency and heterogeneity in our understanding of the ecology of capitalism.