In 463, Mamertus was engaged in a dispute with Pope Hilarius on the question of the privileges of the Bishop of Arles. Pope Leo I had regulated the boundaries of the ecclesiastical provinces of Arles and Vienne: under the latter he left the Dioceses of Valence, Tarentaise, Geneva and Grenoble, but all the other dioceses in this district were made subordinate to Arles. Regardless of this decision and infringing on the rights of his colleague of Arles, Mamertus consecrated in 463 a bishop for the city of Die (Dea). King Gundioc of the Burgundians complained to Pope Hilary of this action, whereupon the latter wrote to Bishop Leontius of Arles on 10 October 463, bidding him summon a synod of bishops from the different provinces to enquire into the matter. In a subsequent letter to the bishops of the Roman provinces of Lyons, Vienne, Narbonnensis I and II and Alpina, he also refers to the matter, and directs them to obey Leontius's summons to a regularly constituted synod. The synod decided against Mamertus, as we learn from another letter of the pope dated 25 February, 464. In this letter, St Hilary declares that Mamertus and the bishop unlawfully consecrated by him should really be deposed; desiring, however that clemency be used, he commissioned Bishop Veranus to inform Mamertus that, if he did not recognize and submit to the regulations of Pope Leo, he would be deprived also of the four suffragan dioceses, still subject to Vienne. The bishop invalidly installed by Mamertus was to be confirmed in his office by Leontius, after which he might retain the bishopric. Mamertus evidently submitted, since we find no subsequent reference to the incident.