(21 Jan.–22 June 1276)
Following the conclave procedure Gregory X had himself prescribed, on the eleventh day after his death the cardinals available met at Arezzo and elected Pierre of Tarentaise, in the upper Val d'Isère (Savoy). Born c.1224, he joined the Dominican house at Lyons c.1240, studied at Paris and in June 1259 graduated master in theology, and held the so-called ‘chair of the French’. In 1259 he collaborated with Albertus Magnus (c. 1200–80) and Thomas Aquinas (c.1225–74) in drafting a rule of studies for Dominicans. His important commentary on Peter Lombard's (c.1100–60) theological treatise known as The Sentences, which had been denounced as unsound in the late 12th century but had been rehabilitated by the fourth Lateran council in 1215, illustrates the transition from the older Augustinian to the new Aristotelian concepts; he also wrote on practical ethics and expounded Scripture. Twice provincial of France (1264–7 and 1269–72), he preached the crusade under Clement IV, while Gregory X named him archbishop of Lyons in 1272 and cardinal bishop of Ostia in 1273. He helped to prepare, and took a prominent part at, the second council of Lyons (1274), and, having been appointed general penitentiary, accompanied Gregory X on his journey in 1275/6 from Lyons to Italy. In 1274 he had preached the funeral sermon at Lyons over the great Franciscan theologian Bonaventura (c.1217–74).
The first Dominican to become pope, the friend of Bonaventura (c.1217–74), Innocent was a man of learning and austere piety rather than political initiative. His election signalled a switch by the curia from Gregory X's policy of seeking to counterbalance the domination in Italy of Charles of Anjou, king of Sicily (1266–85), by making Rudolf I of Habsburg (1218–91) emperor. He immediately (2 Mar.) confirmed Charles in his functions as senator of Rome and imperial vicar in Tuscany, and requested (9 and 17 Mar.) Rudolf, whom Gregory had invited to Rome for his coronation, to postpone coming until his differences with the curia and King Charles were resolved and the question of imperial rights in Romagna settled; meanwhile he should declare void the oaths of allegiance his officials had exacted there. He certainly followed Gregory in placing a new crusade and the peace necessary to its launching in the forefront of his programme, and was successful in reconciling Genoa with King Charles and ending hostilities between Ghibelline (i.e. pro-imperial) Pisa and the Guelph (i.e. pro-papal) Tuscan league. But in resuming Gregory's negotiations with the Byzantine emperor Michael VIII Palaeologus (1259–82) for his participation in the crusade and implementing the church union agreed at the second council of Lyons, he weakened under Charles's continuous pressure. As a result, when informing Michael of Charles's plans to recapture Constantinople by force, he emphasized that it had been violently wrested from the Latins, and on the issue of union demanded that the Greek clergy should take personal oaths in a prescribed form accepting the Filioque and the primacy of the pope. He died, however, just as the envoys carrying these stiff requirements were boarding ship at Ancona.