When the Grande Armée left the shores of the English Channel to fight in the War of the Third Coalition, Rey was given command of the Camp de Boulogne from 1805 to 1808. In the latter year he was named a Baron of the Empire. He served as chief of staff in Laurent Gouvion Saint-Cyr's VII Corps in Catalonia beginning in August 1808. It is likely that he commanded a brigade at the Battle of Ocaña in November 1809. One source stated that Rey led 3,500 men in six battalions. It is probable that his brigade included three battalions each of the 12th Light and 43rd Line Infantry Regiments. At the Battle of Baza on 4 November 1810, he led a brigade from Horace Sebastiani de la Porta's 1st Division of the IV Corps consisting of one battalion of the 32nd Line Infantry Regiment and three battalions of the 58th Line. He fought at the Siege of Tarragona in 1811. Historian David G. Chandler noted that Rey was appointed governor of the fortress of San Sebastián in August 1811. Another source asserted that Rey was defeated by Francisco Ballesteros at Alhaurín el Grande on 14 April 1812. On this occasion he led a force of 3,000 men made up of three squadrons of the 21st Dragoon Regiment, two artillery pieces, and three battalions divided between the 43rd and 58th Line Infantry Regiments. French losses numbered 200 and included both guns.