He was a prominent member of the Council of Basel since 1432 and together with Cardinal Julian Cesarini led the forces that maintained the power of the general councils over the pope's own control of the Church. It was while the council was proceeding that he tended to victims of the plague. He later led opposition to the pope but Cesarini was reconciled with Pope Eugene IV and had a prominent part in the pope's convoked Council of Florence. In 1439 he led the effort to depose Eugene IV and the election of a successor. In 1440 he placed the tiara upon Antipope Felix V and consecrated him as a bishop. This was a misguided attempt at reforming the Church which Aleman believed was vital. Eugene IV was responded to this and excommunicated the antipope while also depriving Aleman of all his ecclesiastical dignities. This also meant that Aleman could no longer be considered a cardinal and he was deprived of the dignities that came with the cardinalate. This occurred on 11 April 1440: he was stripped of Arles as his archdiocese and was stripped of his titular church.