Edward Hugh had a great post about this:
In an interesting development, Spain’s Prime Minister Mariano Rajoy has formally written to the EU asking them for clarification on the legal situation according to the EU Treaty – not on bailouts, or Eurobonds, or ECB bond buying – but on whether or not an independent Catalonia would be a member of the EU and the Euro Area. The issue has arisen because one EU vice president (Joaquin Almunia, Competition Commissioner, who is of course Spanish) has said an independent Catalonia would have to apply for EU membership, while another EU Vice President Viviane Reding (Commissioner for Justice, Fundamental Rights and Citizenship, who is from Luxembourg) stated in an interview with the Diario de Sevilla that existing international law could not be read as implying that a new state that emerged from one of the existing member states would need to apply for entry to the EU.
Naturally Mariano is as unlikely to get the clarification he is requesting on this issue as he is to get it on the issue of direct ESM recapitalisation of the Spanish banks. Following a now well trodden path the EU will attempt to avoid saying anything until the results of the Catalan consultation are know. Should the vote for independence should not obtain a large enough majority, then the EU will back the territorial integrity of Spain, and if it should, then Catalonia will form part of the EU and the Euro Group. If people are worried about an uncontrolled Greek exit, just imagine the panic on the morning after should the Catalan financial system (around 500 billion euros, or 2.5 times GDP) be threatened with going under due to lack of liquidity from the ECB. It doesn’t bear thinking about.