All the nations of South America, except for Guyana and Surinam, are members of ALADI. Its goals are similar to the Unasur. The only two non-South American members are Mexico and Cuba.  Since its inception in 1981, ALADI has achieved very few of its goals and has been eclipsed most recently by the formation of Unasur. This final institution is a specifically South American initiative designed to unite these various processes into one single institution.
Unasur Institutional Structure
Previously known as the Comunidad Sudamericana de Naciones (CSN), it was founded in 2004 by the Declaración del Cusco. It is a 12 nation cooperation treaty designed to propel regional integration efforts forward into the twenty-first century by coordinating dispersed efforts under one central integration authority with its headquarters in Quito, Ecuador.
UNASUR is composed of the nations of MERCOSUR (Argentina, Brazil, Paraguay, and Uruguay), the Andean Community (Bolivia, Colombia, Ecuador, and Peru), Chile, Guyana, Suriname and Venezuela. The union has a combined population of approximately 377 million, an internal market (PIB) of US$1.5 trillion, and a total area of 17 million sq/km, including 27 percent of the Earth's fresh water, 70 percent of its known copper reserves, enough oil and natural gas to last the continent a century, the Amazon rain forest, the fertile pampas, the Andes mountains, and maritime access to the Atlantic and Pacific Oceans and the Caribbean Sea. 
For a region like South America, plagued with internal conflict, weak national political institutions, and the world's worst income disparity, these goals seemed ambitious.
The institutional structure was agreed upon during the 2005 Summit of Brasilia where the Presidents of South America decided that the meeting of heads of state would be the highest level of political contact within UNASUR. These are supposed to occur annually and the location rotates around the region.  This structure was formally ratified in the 2008 Summit of Brasilia.
Meetings of heads of state are the highest level of political contact. The president of country hosting the annual summit is the acting president of the union for one year and the presidents of the previous and following years advise the current president to permit better transitions.
Foreign Ministers make the executive decisions; they are scheduled to meet twice a year to discuss issues of mutual interest and are in charge of preparing the regional agenda for the annual meeting of heads of States. Vicecancillers are in charge of coordinating the respective positions of member countries before the meetings of foreign ministers. Below this level are ad hoc ministerial meetings of specific social sectors such as salud, educación, cultura, ciencia y tecnología, seguridad ciudadana, infraestructura de energia, transportes, comunicaciones, y desarrollo sostenible. 
UNASUR was first created without a formal institutional bureaucracy in order to avoid adding additional layers of administration on top of those already existing in CAN, Mercosur, and ALADI. However, it became apparent by 2007 that an organization dispersed over a continent without any sort of central administration was not practical. To correct this, an office of Executive Secretary was created with headquarters in Quito, Ecuador and a regional parliament will be located in Cochabamba, Bolivia.
The position of Secretaria General was offered to Rodrigo Borja, a former president of Ecuador, in 2007. Borja accepted and began drafting a constitutive charter for the union that would have combined the subregional blocks of CAN and Mercosur into Unasur.
His proposal also called for a strong executive branch which would be in charge of setting the regional agenda and ensuring compliance. In Feb 2008, Borja made clear in a private interview with the author that he would not be part of just another layer of ineffective state bureaucracy. Although his draft was submitted to the presidents of each country in 2007 he had not received a response at the time we spoke with him. 
One day before the May 2008 Summit of Brasilia, he publically rejected the offer to serve as Executive Secretary because the presidents rejected his proposal to create an agile institution capable of achieving the ambitious goals of integration. Instead, it was decided to create a forum of 12 representatives from each member nation as well as the Executive Secretary while reserving maximum authority for the presidents of each country. 
Also involved in the direction of Unasur are the Presidente del Comité de Representantes Permanentes del MERCOSUR, the Director de la Secretaría del MERCOSUR, del Secretario General de la Comunidad Andina, the Secretario General of de la ALADI, and the Secretaría Permanente de la Organización del Tratado de Cooperación Amazónica to provide the necessary staff to carry out the mandates approved by presidents and foreign ministers.
By Nathan Gill - Southern Affairs
By Nathan Gill - Southern Affairs
 Alfaro Espinosa, Silvia. Personal Interview. Lima 27 Feb. 2008.  The exclusion of Mexico from Unasur will be analyzed further on.  Velasco, Gema. "La Comunidad Sudamericana, ¿ilusión o realidad?" Americaeconomica.com - 10 Dec 2004.  The 2007 Unasur Presidential Summit scheduled to be held in Colombia was postponed until Mar 2008. It was then postponed and relocated to Brasilia after the diplomatic crisis brought about by the Colombia Ecuador FARC dispute.  Declaración del Cusco, Articles 7-11, 8 Dec 2004  Borja, Rodrigo. Personal Interview. Quito 8 Feb 2008.  El Universal Caracas. "Renuncia de Rodrigo Borja empaña inauguración de UNASUR" 22 May 2008.  Declaración del Cusco, Article 3, 8 Dec 2004