Since 2000, an international reform process has been underway to reduce the negative impact of tax havens. This article analyses whether the reform period has favoured protected tax havens, i.e. havens with strong connections to the United Kingdom, the European Union, the United States and China, relative to tax havens without such connections. Using an implicit difference-in-difference approach with heterogeneous treatment effects, the author shows that portfolio investment in protected havens increased significantly more than in unprotected havens in the period 1997 to 2015. In other words, through their associated territories, some of the most powerful countries in the world seem to have cornered the market for tax haven services during the reform period. This may make further reform more difficult.