During previous work in this laboratory, Murphy, Veerkamp, and Leland developed a relationship to describe the charge transfer process between a semiconducting catalyst and a chemisorbing gas. The expression derived states that for certain pressure ranges of the adsorbing gas, a plot of In a versus Q for a p-type semiconductor will be linear with a slope of -1/1-Xs(e/K). The Xg term represents the fractional extent of electronic inhomogeneity in the catalyst in terms of distinct surface and bulk regions. The first part of this work evaluated the behavior of cupric oxide relative to the theory. The results show cupric oxide to remain virtually homogeneous electronically during both anionic and cationic adsorption. Photodesorption of carbon dioxide from zinc oxide was studied as a possible mechanism for the photocatalytic effect observed for the oxidation of carbon monoxide over zinc oxide. The results of the study indicate that ultraviolet light of energy slightly greater than the ZnO band gap energy (same wavelength as the light which caused the photocatalytic effect) does not enhance the desorption of the CO2. As a consequence it is concluded that the photodesorption of carbon dioxide is not explicitly involved in the photocatalytic mechanism.