International commercial arbitration does not exist in a legal vacuum. There are several laws and rules that may be applicable to it. One of them is the law governing the arbitration agreement. Determination of the law governing the arbitration agreement in the sense of its substantive validity is one of the most interesting questions in international commercial arbitration as it is still not completely clear. There are two possibilities that can be considered as regulating the substantive validity of arbitration agreement – the lex arbitri or the lex causae of the the principal contract. This article aims at analyzing both of these possibilities and tries to reach a conclusion on which of them should be applied. Firstly, the specific nature of the arbitration agreement is briefly dealt with. The question of substantive validity may most likely arise at two stages of arbitration – at the beginning of the arbitral proceedings and at the award stage. In other words, the question of substantive validity may be considered by both arbitrators themselves and by courts. This article will analyze the possibilities of assessing the law applicable to substantive validity from the point of view of both arbitrators and national courts. It will take into account applicable international conventions as well as several national regulations.