Serbia and Montenegro's full cooperation with the Hague war crimes tribunal is one of the key issues that should be resolved, Solana told reporters in Belgrade after meeting the state union's President Svetozar Marovic.
Solana noted that the country's cooperation with the Hague tribunal had improved significantly, but that Brussels expected Serbia and Montenegro to take the final step and extradite Bosnian Serb wartime military commander Ratko Mladic.
Solana also met for talks with Montenegrin Premier Milo Djukanovic and Serbia and Montenegro's Foreign Minister Vuk Draskovic.
On Tuesday evening, the EU representative met Serbian President Boris Tadic and Premier Vojislav Kostunica.
The Serbian Government issued a statement after the meeting, saying that the talks focused on the state union's aspirations to join the European Union and that "this topic should be given priority over all other topics in the country, particularly over a possible referendum (on independence) that has been announced by Montenegro."