In March 2012, as part of the research on Trust Building in Nuclear Worlds, the researchers on the project conducted, in partnership with the Getulio Vargas Foundation (FGV) in Brazil, a Critical Oral History workshop on the origins of Argentina-Brazil nuclear cooperation. Together with other evidence collected over the duration of the project, the exercise has made important contributions toward a new understanding of this important case. Concisely put, the evidence now shows that this relationship did not constitute a nuclear rivalry that was transformed into cooperation, as the existing literature deems it to have been, but instead would be better characterized as a case of a political rivalry that never developed into a nuclear rivalry in the first place. The reasons for this were multifarious, but included, crucially, the personalities of key-figures and inter-personal relationships between scientists and bureaucrats in the two countries' nuclear establishments. The full transcripts will shortly be available for researchers and the video for the meetings will become a short documentary film about the development of the Argentina-Brazil nuclear relationship and the Critical Oral History methodology.