The mechanism of the effect of adsorbed oxygen on photoluminescence (PL) of ZnO powders and ZnO/Si ALD films in the UV–VIS regions was studied, simultaneously with the in-situ UV photoelectron spectroscopy (UPS) and mass spectrometry (MS) measurements. We have found a drastic (up to 10 times) increase of the exciton PL along with a slight (by ~ 1.5 times) increase of green PL induced by thermo-reducing of ZnO surface in H ₂ or CO atmosphere or by a UV–VIS photo-reducing in the vacuum. The reaction products H ₂O, CO ₂ and photo-desorbed O ₂ were registered by the MS. According to UPS (8.43 eV), the change in PL is accompanied by a significant change in the surface dipole value δ without shift of the Fermi level E _F or of the band bending V _S. We believe that the slow surface states interact with the adsorbed oxygen and thus create a surface 2D-quantum well whose internal field destroys the excitons. The internal fast surface states not interacting with the slow ones provide a pinning of the Fermi level and the stability of the band bending value. The adsorbed oxygen also affects the surface defects thus reducing the VIS luminescence of ZnO.