This paper examines the likely impact of a proposed formula apportionment system for corporation tax in the EU on the inbound investment of US multinational companies. We pay attention to tax planning strategies that may be employed by US multinationals and investigate whether effective tax rates in Europe of US companies differ from those of European companies. The proposal is for an optional system: we estimate the extent to which both European and US companies would be likely to choose it taking into account their existing structures and future investment incentives. The relative position of US and European companies depends crucially on the taxation of foreign passive income.