Debates on EU external relations among scholars and policy-makers have often lamented the lack of coherence and inability of the EU to 'speak with one voice'. This article, by contrast, focuses on the ability of the EU to use diplomatic capacity to understand the preferences and domestic politics of third countries. This aspect of EU external relations has been somewhat neglected in both academic and policy debates thus far, and the article uses the case of EU engagement with China on climate to illustrate its importance. EU-China engagement on climate change has resulted in a range of bilateral cooperation activities, but has delivered less for the EU in terms of developing a better European understanding of the preferences and domestic politics of climate change in China. The article further discusses how particular institutional challenges have constrained the EU's climate diplomacy, including vertical and horizontal fragmentation and a lack of institutional capacity on the part of the EU.