We report on measurements of photodesorption from ZnO powders and single crystals, carried out for the first time by quantitative mass spectrometry under controlled uv radiation.1 Contrary to the widely held assumption as to the role of oxygen in photodesorption,2,3 our measurements indicate that only neutral CO2 is photodesorbed from ZnO. Strikingly, we found that CO2 is also the main photodesorbed species from stainless steel and CdS.The linear dependence of the photodesorption rate on the incoming photon flux is indicative of a true photodesorption effect; in addition, the spectral response of the photodesorbed CO2 also shows that a quantum process occurs. The sharp cutoff of the photodesorption signal, when the incoming photon energy falls below the ZnO band‐gap energy (3.2 eV), strongly supports a substrate‐dependent model of photodesorption.Measurements of the simultaneous time dependence of CO2 photodesorption and photoconductivity changes establish the role of the CO−2 ’’ion molecules’’ in the photodesorption process, which takes place when CO−2 is neutralized by photogenerated holes. This process is shown to have an activation energy of 0.25 eV as found from a measurement of the temperature dependence of photodesorption. A calculation of the cross section for breaking the chemisorption bond by the holes drifting to the surface yields a value of 10−21 m2 (10−17 cm2). Models which explain the photodesorption as well as the chemisorption process are discussed with further supporting experimental evidence from Auger electron spectroscopy measurements.