Abstract Liberius is often viewed as Athanasius’ ally in the mid-350s, but his own writing, contemporary testimony, and the pagan historian Ammianus Marcellinus suggest otherwise. Rather than defending Athanasius at the councils of Arles and Milan and in Constantius II’s court, Liberius attempted to establish acceptable conditions for peace in the Church by trading Athanasius’ excommunication, which he saw as justified by Athanasius’ refusal to comply with traditional ecclesial-judicial procedures, for doctrinal concessions from certain Eastern bishops whom Liberius suspected of sympathizing with heretical doctrine.