The resistance of thin cobalt (Co) and niobium (Nb) films was monitored during their exposure to hydrogen or carbon monoxide for 1000 s, at a fixed pressure, and also during the removal of the gas from the surface. In an adsorption-desorption cycle the resistivity changed in a "saw-tooth" fashion, similar to the changes observed in traditional gravimetric studies. The resistivity increase caused by the adsorbed gas seems to be directly related to weakly adsorbed states and the roughness of the film surface. The surface roughness decreases the magnitude of the adsorption-driven resistivity change in such a way that rough-thin Co films show a similar resistance change to that for smooth-thick Nb films. The roughness of the films depends on the deposition technique used, and it was determined by low-angle X-ray diffraction and scanning tunneling microscopy.