In July 1808 Major Lefebure and Jones were selected to serve as the two assistant-commissioners under General Leith, appointed military and semi-diplomatic agent to the junta of the northern provinces of Spain. Jones was attached to the army of the Marquis de la Romana, and conceived a great affection for its commander. Towards the end of the year Leith was ordered to take command of a brigade and to select an officer to succeed him as commissioner. Leith offered to appoint Jones, but Jones declined, although the high rate of pay was tempting, on the ground that his youth and want of rank would deprive his advice of its proper weight, and he asked instead to join the army. Leith at once appointed him his acting aide-de-camp. Jones continued to act in this capacity until after the skirmish in front of Lugo, when he was ordered, as an engineer officer, to assist in blowing up the bridge over the Tamboya, and was employed with his own corps during the retreat to Corunna. On his arrival in England Jones resumed his staff appointment at Woolwich, and on 24 June 1809 was promoted first captain. On the 9th of the following month he was appointed brigade-major to the engineers under Brigadier-General Fyers, to accompany the expedition under the Earl of Chatham to Walcheren.