La0.8Sr0.2Co1-xNixO3-δ perovskite films prepared by the radio frequency sputtering technique in a wide range of Ni content (x≤0.6) were studied. The microstructure of the films determined by scanning electron microscopy showed the changing of grain size after heat treatment at different temperature for 5 h. The effect of Ni doping on the sensitivity to CO was studied. The La0.8Sr0.2Co0.6Ni0.4O3-δ film treated at 700 °C for 5 h shows the best improvement of sensitivity to CO. The CO–O2 reaction on the surface of film can be indirectly probed by electron spectroscopy for chemical analysis. The sensitivity dependence affected by grain size is also investigated. The combination of binding energy to the adsorbed oxygen and the connecting condition of the adjacent grains would dominate the sensing property. The results, show the La0.8Sr0.2Co0.6Ni0.4O3-δ film treated at 700 °C for 5 h yields the maximum sensitivity of 10.1% in 50 ppm of CO. © 1999 American Vacuum Society.