We have employed UHV surface sensitive techniques together with CO and oxygen adsorption and reaction to link the chemical, electronic and structural properties of ultra-thin Pd films vapour-deposited on Ru(100). At low Pd coverage the properties of the Pd overlayer are considerably modified relative to the bulk-like properties exhibited by thick Pd films. A Pd coverage of 6 ML seems to mark a critical transition to bulk-like Pd behaviour. The strongest modifications in the Pd overlayer occur for coverages up to 2 ML and are exemplified by a reduction in the CO desorption temperature, a low DOS at Fermi level, E(f), and a lower activity toward CO oxidation.