This article explores the impact of democratic transitions in Southeast Asia on regional co-operation, and the relationship between this process and the development of a non-official regionalism. Until now, regionalism in Southeast Asia has been essentially elite-centred and politically illiberal. The emergence of the Association of Southeast Asian Nations was founded upon the common desire of its members, which had by then retreated significantly from their postcolonial experiments in liberal democracy, to ensure regime survival. This orientation was further institutionalised by ASEAN's doctrine of non-interference, which helped to shield its members from outside pressures towards democratisation. But with democratisation in the Philippines, Thailand and more recently Indonesia, the ASEAN model of elite-centric regional socialisation has been challenged. The civil society in the region demands greater openness in Southeast Asian regionalism. The article proposes a conceptual framework for analysing the relationship between democratisation and regionalism, with the key argument being that the displacement of traditional patterns of regional elite socialisation has been offset by potential gains such as advances in regional conflict management, transparency and rule-based interactions. But the realisation of a more 'participatory regionalism' in Southeast Asia faces a number of barriers, including obstacles to further democratisation, the continued salience of the non-interference doctrine and the diminished space for civil society in the wake of the 11 September terrorist attacks.