MERcado COmain del SUR. Argentina, Brazil, Paraguay, and Uruguay formed this customs union established under the 1991 Treaty Of Asunciean. In 1996, Bolivia and Chile joined it as its associate members. Following the formation of the European Community’s Treaty Of Rome, interMercosur trade is duty free, while levying a common external tariff of 0 to 20 percent on non-member countries. Known as Mercosul in Brazil.