 From 1962 to 1965, Tisserant attended the Second Vatican Council and sat on its Board of Presidency.As Dean of the College of Cardinals, he was the celebrant of the Mass "coram Summo Pontifice" at the opening ceremony of the Council, before the Pope delivered the Council's opening address and the other rites specific to the opening of a general council were performed.He is said to have participated in negotiating a secret 1960s agreement between Soviet and Vatican officials that authorised Eastern Orthodox participation in the Second Vatican Council in exchange for a noncondemnation of atheistic communism during the conciliar assemblies.