Following the 1955 "Revolución Libertadora", a military coup which ousted Perón, Alonso was detained, before being unexpectedly released on 25 June 1956. Apparently that was an error, and the "junta" tried to capture him again, but Alonso managed to join Perón in his Venezuelan exile. He then participated to the negotiations between Perón and Rogelio Julio Frigerio, who represented presidential candidate Arturo Frondizi of the UCRI. These negotiations resulted in Perón's decisive endorsement of Frondizi in the 1958 presidential elections.