There is extensive literature on the implications of the investment treaty regime for public interest regulation. However, little systematic attention has been paid to the capacity of states to make investment treaties and the obligations states can or cannot agree to under those treaties in light of their public interest obligations. This issue is of fundamental importance because both the duty to regulate in the public interest and the capacity to conclude investment treaties are determined by constitutions and general international law. Does a state the constitution of which requires it to act within its constitution and general international law in the public interest have the capacity to conclude investment treaties that directly or indirectly prohibit the performance of that duty? With reference to Ghana, this article argues that the legal source and public purpose of the State’s powers prevent it from concluding agreements that directly prohibit public interest regulation or indirectly achieve that effect. Accordingly, it is suggested that the express and implied limitations on the duty to regulate in the public interest placed on investment treaty making powers of the State must inform the making and interpretation of investment treaties.