The world’s third largest trading group, comprised of Argentina, Brazil, Paraguay, and Uruguay. Bolivia and Chile are associate members. The four main countries created Mercosur in March 1991 when the Treaty of AsunciÃ³n was signed. The group took effect on December 31, 1994. Mercosur was established with the purpose of creating a common market and customs union among the countries using the economic cooperation between Argentina and Brazil since 1986 as a model. Mercosur also established common external tariffs and adopted a common trade policy in relationship to other countries.