The coadsorption of alkalis (Na, K) with O on Cu(1 1 1) and Ni(1 1 1) surfaces was studied by vibrational measurements. We found that doping the Cu(1 1 1) surface with small amounts of alkalis reverses the energetic conditions for oxygen adsorption so as to render energetically favorable the population of subsurface sites. Such mechanism is not effective for the alkali-modified Ni(1 1 1) surface.