In international investment law, there is an instrinsic conflict between the host states right to regulate and the substantive legal protections afforded to foreign investors under international law. By exploring the theoretical fundaments of proportionality analysis, this study shows how the structure of proportionality balancing induces investor state tribunals to throughly weigh all competing interests, hereby enabling arbitral tribunals to make a rational finding on which of the competing interests prevails in the individual case at bar. These effects are the result of the procedure of proportionality balancing. The structure of proportionality analysis can therefore improve the overall foreseeability of investor state tribunals law application, which may ultimately contribute to enhance the legitimacy of investor state arbitration in the European investment protection law.