In the late 1990s, European experts feared a stalemate in the EU accession negotiations with Central and Eastern European (CEE) countries because of the expected high costs of their compliance with the EU’s environmental legislation. This fear was not borne out, however, and it is now undoubted that EU assistance has enhanced environmental capacity-building and knowledge in these countries. Nevertheless, some reports have emphasised misallocations, planning failures and mismanagement of EU funds, characterised as the unsustainability of EU-funded environmental projects in a number of CEE countries. Why have some EU-funded projects been more sustainable than others? Which features have had an effect on their sustainability? The article investigates the relationship between European and domestic actors involved in EU-funded projects in municipal waste management in Hungary and Poland in the years 1998–2013. The main findings show that when decision-making participation was horizontal and cooperative between the EU and domestic actors, EU assistance to municipal waste projects was more successful and long-lasting in helping them comply with EU legislation.