Selective surface is used for the utilisation of solar energy by photo-thermal devices. For this purpose CuO and CrO3 films were prepared and their optical properties like reflectance, emittance and absorptance were studied. The selectivity of different samples were calculated from the measured data. Film thickness was measured with a high resolution micrometer. The optimum thickness for maximum selectivity was determined from the thickness vs. selectivity plot. A selective surface of CuO showed a maximum selectivity of 8.08 at thickness 0.011 mm and CrO3 showed a maximum selectivity of 19.6 at thickness 0.015 mm. The result is better than the results obtained by earlier researchers.