We report measurements of CO2 photodesorption and associated surface-conductivity changes in ZnO. The measurements show a one-to-one correspondence between the rates of those two quantities. The rates of both CO2 photodesorption and surface-conductivity changes are initially fast, becoming much slower with illumination time. A calculation, based on the assumption that this time rate of change is related to a change in the degree of band bending during photodesorption, shows excellent agreement with the experimental results. This model is further supported by Auger-electron-spectroscopy measurements. The cross section for photodesorption is estimated to be ∼ 10-17 cm2.