An important aspect of administrative decision-making is establishing the relevant facts by the administrative body during the administrative decision-making and by the administrative judge during the appeal proceedings. Despite this importance the process of establishing facts in administrative law and the assessment by the judge of the inquiry of the facts by the administrative body, is a neglected area of juridical research. In some situations the administrative body might expect that the citizen, who applies for a permit, gives the relevant information on its own initiative. In other situations one would expect that the administrative body is responsible for establishing the facts. The subtle balance between both parties involved becomes even more complex in the situation where third parties whose interests are affected by the decision get involved in the administrative procedure. In this complex situation the judge, who has to assess the decision once it has been made, copes with several questions. Which party has to provide evidence for the disputed facts? And in what circumstances has the judge to use his own investigatory powers to uncover the truth? In this paper we try to find an answer to these questions, using socio-legal methods. We tested three different types of variables that might explain the activity of the judge. The characteristics of the disputed decision, the arguments and characteristics brought forward by parties and finally the characteristics of the organizational structure might affect the choice of the judge to use his investigatory powers. We conclude that legal arguments hardly affect the activity of the judge. The use of investigatory powers seems to be influenced by the parties and the organizational environment in which the judge operates.