Pope Clement IV (Guy Foulques) fell ill on the Feast of S. Cecilia (22 November), and died at Viterbo on 29 November 1268. He had governed the Church for three years, nine months, and twenty-four days. The See of Peter was vacant for two years and nine months. Cardinal Simon de Brion came from France to attend the Conclave, which took place in the Episcopal Palace, next to the Cathedral of S. Lorenzo in Viterbo. He was the senior cardinal-priest. Around Pentecost of 1270 (1 June), Cardinal Simon and Cardinal Riccardo Annibaldi of S. Angelo had to leave the Conclave and retire to their residences for the sake of their health. On 22 August 1270, he was one of the signatories to the letter of protest sent by the Cardinals to Raynerius Gatti, Captain of the City of Viterbo, to cease and desist from their harassment of the Cardinals and their suites. He was one of the cardinals who signed the electoral compact on September, 1270, to leave the election of a new pope to a committee of six, promising to accept the committee's decision. He was not, however, one of the six cardinals elected to the Compromise Committee that selected Archdeacon Teobaldo Visconti as pope on 1 September 1270. The newly elected pope was not present in Viterbo, but was serving on Crusade with King Edward I of England. He arrived in Italy on 1 January 1271, and travelled to Viterbo, where he arrived early in February. He accepted the election, and chose to be called Gregory X. He and the Curia travelled to Rome, arriving on 13 March. On 19 March he was ordained a priest, and on 27 March he was consecrated bishop, and then crowned by Cardinal Giovanni Gaetano Orsini.