Of the remaining five entities - Albania, Montenegro, Bosnia and Herzegovina, Serbia and Kosovo - that want to join the EU, Mr Rehn had the most to say about Bosnia and Herzegovina.
The war-torn country was recently given an ultimatum by the EU and the US to sort out internal problems between Bosnian Muslims, Croats and Serbs by 20 October.
Defining the country as of "paramount importance for the region and for the European Union," Mr Rehn said that Bosnia and Herzegovina could only consider an application for EU membership once it "can stand on its own two feet."
"No quasi-protectorate can join the EU," he said, spelling out that the Office of High Representative would have to be closed down first. The post was created as part of the peace deal that ended the 1992-1995 war in the country, and can only be closed after a positive international assessment.
Meanwhile, the Serbian government, which is being pushed to arrest two war crimes suspects from the 1990s, was praised for being "stable" and "demonstrating" a high degree of consensus on EU integration as a strategic priority."
But even as the EU tries to bind all of the countries of the western Balkans and Turkey ever more closely through political and economic ties and the promise of eventual membership, there are continuous doubts about whether it has the political appetite to go through with another large round of expansion.
Apart from Croatia, strongly supported by Germany and where EU membership is virtually assured, internal EU question marks remain over the rest.