Transferred to Spain, he was acting commander of a light infantry regiment in the VI Corps for a time before being elevated to colonel in September 1810.

The unit was part of Martial Bardet de Maison-Rouge's brigade in Julien Augustin Joseph Mermet's division of Marshal Michel Ney's VI Corps.

With his 2,000-man brigade, Deconchy joined the 16,000 troops under Maximilien Sébastien Foy who were evacuating the northern coast of Spain due to the British victory at the Battle of Vitoria.

During that action, Deconchy's brigade held the walls of the town and easily repelled two attacks by the 1st King's German Legion Light Battalion under Christian Friedrich Wilhelm von Ompteda.

[3] As he fell back toward France, Foy sent Deconchy's brigade and every artillerist he could spare to defend San Sebastián which was commanded by Louis Emmanuel Rey.