Attributing a wrongful act to a state in an investor-state arbitration is governed by public international law. However, understanding whether such an act was undertaken by an agent of a state or other entity exercising elements of governmental authority, figuring out the legal nature, effects, scope and legality of such acts and comprehending whether they were taken from the point of public authority requires application of domestic administrative (or wider public) law of the respondent state. Administrative law makes applied public international law provisions operational. In order to evaluate the occurrence and frequency of administrative law challenges in investment arbitration, the author carried out a survey amongst ICSID arbitrators. The results of the survey, presented in the paper, confirmed the hypotheses that it is not seldom in investor-state arbitrations to come across complicated administrative law issues, the resolution of which is a prerequisite for deciding an arbitration case; that a majority of arbitrators in investor-state arbitrations did not specialize in administrative law; and that currently applied mechanisms of handling such complex administrative law issues in practice are not satisfactory.