The European Union is bound to a duty of consistency between its internal policies and external action and a duty of sincere cooperation with its Member States. When addressing the status of the OECD Transfer Pricing Guidelines and the OECD arm’s length principle, the European Commission has adopted two different views: one in matters of domestic income taxation of third states and its Members, and another dealing with EU State aid. This article addresses the consequences of such behaviour. It concludes that the aforementioned duties require the European Commission to deem the OECD Transfer Pricing Guidelines as the only possible implementation of the arm’s length principle when addressing State aid transfer pricing matters. The European Commission can, however, still decide to engage Member States to further its positions on the matter in order to develop the understanding of the arm’s length principle and the OECD Transfer Pricing Guidelines at the OECD level.