The photostimulated reactions of small molecules, i.e., O-2 photoadsorption and photodesorption, H-2 and CO photooxidation, and CO2 photodecomposition on alkali halides (alkali halides) surface are discussed in this work. Most of 19 alkali halides show the activity in the reactions mentioned above. Three spectral regions of alkali halides stimulation by light are distinguished in relation to kinetics of oxygen photoadsorption and reactivity of adsorbed oxygen species. They are in correspondence with: (i) bulk exciton absorption bands; (ii) extrinsic and intrinsic bulk and surface absorption bands and (iii) color centers absorption bands. In the latter case the activity of alkali halides arises after pre-excitation in exciton bands, which leads to sensitization of alkali halides to visible light. The H-2 and CO photooxidation on alkali halides is two-step photoprocess involving some excited states of adsorbed oxygen. The latter is quenching by gaseous oxygen as it is shown for hydrogen oxidation over KBr. The photodecomposition of carbon dioxide is treated as CO2 dissociative adsorption with CO evolution into the gas phase. (C) 2000 Elsevier Science B.V. All rights reserved.