This paper measures the effect of state-level supermajority requirements for tax increases on tax rates. Unobserved attitudes towards taxation tend to influence both the adoption of supermajority requirements and tax policy. Consequently, one cannot distinguish between the effect of these requirements and their correlation with these unobserved attitudes. A model is presented in which legislatures controlled by a pro-tax party adopt a supermajority requirement to reduce the majority party agenda control. The propensity of pro-tax states to adopt supermajority requirements results in an underestimate of the true effect of these requirements on taxes. To correct this identification problem, the paper first uses fixed effects to control for unobserved attitudes and then employs instruments that measure the difficulty of amending state constitutions. The paper concludes that supermajority requirements have significantly reduced taxes.