Using CORTAX, a computable general equilibrium (CGE) model designed to assess the economic impact of corporate taxation, we examine the possible economic impacts of uncoordinated and coordinated changes in national corporate tax rates among a group of economies (the EU) that are tightly associated through international trade and investment. The aim is to contribute to the ongoing debate about the desirability, modality and likely impact of alternative policy solutions to the challenges posed by tax competition and aggressive tax planning. Corporate income tax rates can generate substantial responses within the implementing country as well as beyond its own borders. Harmonisation of CIT rates would likely involve winners and losers, and as such, may be best pursued gradually and as part of a broader package of corporate tax reform.