International treaties are source of law, and their relevance in the current configuration of Customs Law is crucial since they are derived in adaptation to various agencies and international commitments. The reason for their existence and content, is attributed to the need for regulatory provisions that lower barriers to the free movement of goods. Thus, the taxable event in the General Import Tax Law that gives rise to the obligation to pay is the introduction of goods to the country. Related to the obligation to pay is the time variable. If the taxpayer wants his goods indefinitely in Mexico he must pay. unless they are exempt. Paying the tribute implies a claim from the taxpayer to meet the variable time. In view of the foregoing, all legal presumptions requiring the payment of the General Import Tax are to be derived from the introduction of goods into the country and its residence time should be indefinite.