The paper discusses systematic barriers for an enhanced institutionalisation of integrated environmental assessments at the level of European Union (EU) environmental policy-making. It draws from recent experiences of a programme that aimed to provide useful assessments as a basis for more rational decision-making in this field. Two complementary explanations are given to account for the difficulties the programme met in achieving its goals. First is the way science's role in the policy process depends on the degree to which a policy problem is well structured, that is, the degree to which it bears a consensus both on relevant values and relevant knowledge. The second explanation is an institutional one that emphasises the difficulties of establishing a social relationship between 'providers' and 'users' of scientific assessments at the EU level. © Beech Tree Publishing 2006.