Deliberation, as a mode of interaction based on the logic of reason-giving argumentation, is a core feature of the European Union institutions. Yet only few studies have explored the conditions that make deliberation possible in practice. This study examines the institutional determinants of deliberation within joint parliamentary committees (JPCs) – longstanding instruments of EU enlargement policy. The empirical analysis reveals a dynamic relationship between ‘deliberation’ and ‘debate’ as extreme modes of interaction that co-exist within the same setting. It also suggests that deliberation is a product of participants’ constant efforts to maintain equal power relations and low issue-area sensitivity. This study provides new evidence on deliberative politics at the EU level. In addition, it highlights the role of inter-parliamentary deliberation as a catalyst for political co-operation and policy co-ordination, at a time of intensifying enlargement fatigue and growing Euroscepticism both at home and abroad.