Abstract: Lex mercatoria is a body of a-national rules, which can be used as the applicable law to an international contract, provided that disputes between the contracting parties shall be settled by arbitration. To analyze the application of lex mercatoria are clarified, first, issues relating to the limits of choice by the parties of the applicable law and the concept of lex mercatoria and, then, are treated two possible situations: the situation in which the contracting parties have chosen lex mercatoria as the applicable law to their contract, either as the only applicable law, or as complementary rules to a national law; the situation in which the parties have not chosen the applicable law and the arbitral tribunal applies lex mercatoria for the merits of the dispute.

Keywords: lex mercatoria, international arbitration, applicable law