European energy policy is characterized by a complex allocation of authority between the European Union and its Member States which results in an intricate interplay of regulatory competence. Knowing the difficulties European countries face in coordinating and proposing common solutions in the area of energy, there is the urgent need to question the legal foundations underlying the decisionmaking process. Institutional paralysis, low reactivity to events and changes as well as systematic political horse-trading across all questions call for an alternative framework allowing some pioneering Member States to promote ad hoc common policies escaping the formal and procedural requirements of EU law. Our paper assesses the legal feasibility of short-run differentiation by means of partial international agreements inspired by the Schengen regime, namely entirely outside the EU framework. The key challenge from a legal point of view is to assess the substantive compatibility of such agreements in energy with the existing rules of the Union. Short run differentiation in energy cannot indeed be assessed at a high level of generalities. We therefore take two areas where legally-binding coordination at the sub-Union level is often called for: nuclear policy and gas security of supply. The possible substantive content of such cooperation is derived from the economic and political literature before legal feasibility is assessed. Our findings suggest that the scope for such agreements is limited for security of gas supply whereas it could be an improved cooperation device in certain areas of nuclear policy.