This study applies the concept of social capital to participatory slum upgrading, specifically the Baan Mankong (“secure housing”) programme in Bangkok. The Baan Mankong programme uses community participation to meet the housing needs of the urban poor, with financial assistance from the state. Since starting in 2003, the programme has drawn international praise, and is being scaled-up nationally, yet few studies have examined its social and institutional outcomes, focusing rather on the physical outputs. This study tries to fill this gap: as a programme that aims to be about “more than just houses”, attention needs to be paid to its impacts on both horizontal and vertical associations to determine whether it really offers an increased role for the urban poor in governance. A qualitative approach was taken, using semi-structured interviews, participant observation and discussion groups in four case-study communities. The analysis is structured on three levels: intra-community ties, inter-community ties, and state-community linkages, representing bonding, bridging and linking social capital respectively. At the level of intra-community relations, the study finds that upgrading had positive but temporary effects on collective action. Community leadership can be a uniting or dividing force, determining whether collective activities are sustained. Slum networks, representing inter-community relations, are essential for scaling-up upgrading through learning-by-doing. There is scope for cooperation between different networks for unity in negotiations with the state. With regard to community-state linkages, bureaucracy can still be a barrier to effective cooperation, and trust in officials remains low. This study focuses on those at the core of the upgrading process, and offers suggestions for ensuring that collective action provides the best results for both the urban poor and the state. Social capital is a valuable resource for the poor, when the formation of horizontal and vertical associations is actively promoted. As participatory development becomes the new paradigm and the poor increasingly take the initiative in ensuring their needs are met, a fuller understanding of Thailand’s experiences can help shape housing and community development policies elsewhere.