The scope of binding mandatory provisions of procedural and substantive law seems to be one of the most important problems of arbitration, especially international arbitration. It is connected to various other concerns, e.g. the law applicable to various elements of arbitration, mainly the arbitration agreement, but also arbitrability. Poland, with its Private International Law of 2011, is a party to the European Convention on Arbitration and the New York Convention. It attempted to resolve this issue in a clear manner. Even a confirmed indication of the law governing arbitration – and the arbitration agreement to be specific – does not necessarily signify that there is unambiguity as to which substantive, procedural and public law provisions are mandatory and thus binding upon a court of arbitration. The answer to this question is of paramount importance for the parties as the violation of mandatory provisions of law can lead to setting aside an arbitral award or to denying its enforcement or recognition. This paper discusses these issues from the perspective of the arbitration background of the Republic of Poland with its relatively new arbitration law and private international law.