The establishment of criminal regulations of both substantive and procedural law based on instruments of international law, known as the internationalisation of criminal law, is not a new phenomenon. The novelty of this phenomenon is the scale at which it has spread. It is implemented through various mechanisms, ranging from the cooperation between States, harmonization and approximation, to the integration of various criminal law systems. Harmonization presupposes a mechanism for the interaction of the various criminal law systems; in this sense, harmonization is an imperfect process, as the systems retain some of their differences. International conventions delineate new criminal offences, commonly called ‘treaty crimes’. In this paper, we propose to look at criminal harmonisation via the lens of treaty crimes. Our hypothesis is that the national legislator rarely deviates from the definition provided by the international standard, but does so on occasions where different obligations overlap or in cases of non-mandatory criminalization. To that purpose, we will look at how several international treaties have been applied in Argentine law, as well as how the obligations to criminalize have been implemented.