When the surface of a solid semiconductor or dielectric metal oxide (or other) specimen is preirradiated, the solid often retains its photochemical activity after termination of irradiation through formation of long-lived surface-active adsorption centers. This effect has two origins, viz., the so-called Kugel'sberg memory effect and the Solonitsyn memory effect. The former denotes preirradiation in the presence of the adsorbate molecules, whereas the latter refers to preirradiation in vacuo followed by subsequent introduction of adsorbate molecules into the reactor. This article reports results of detailed studies on the Solonitsyn memory effect in gas/solid heterogeneous systems with respect to photostimulated adsorption (i.e. reductive or oxidative adsorption) of molecular oxygen, molecular hydrogen, and methane on the surface of a dielectric metal oxide such as zirconia. The memory effect has been quantified for several metal oxides and alkali halides by means of an experimentally determined postadsorption memory coefficient, η(t), which defines the fraction of long-lived photoadsorption centers with respect to the total number of both long-lived and short-lived surface centers of photoadsorption generated for a time of irradiation, t. A simple model is proposed to explain the experimental data.