Mass spectrometry in the channel of the reactor at 10^-6-10^-3 mm Hg has been used to study the effect of light on the adsorptive capacity of ZnO. The sign of the effect, photoadsorption (PA) or photodesorption (PD), does not depend on the incident wavelength λ, but is determined by the conditions of previous treatment of the adsorbent. We have determined the cross section for the capture of O₂ by PA centers [σ′ = (1.3-2) × 10^-18 cm²] and by the centers causing release in PD [σ″ = (0.4-0.6) × 10^-18 cm²]. According to thermal desorption results, the forms of photoadsorbed and postadsorbed O₂ are identical with the forms of chemisorbed O₂ on samples which have not been irradiated.