The use of semiconductor materials as photocatalysts for cleaning gas processes has received increasing interest in the last decade. In order to make more active catalysts, it is worthwhile to investigate the main processes influencing photocatalytic reactions in more detail. One of these is the process of coadsorption of the reaction components on the catalyst surface under irradiation. It is shown that photoassisted NO adsorption can serve as a model system in order to investigate the influence of irradiation intensity and temperature on the adsorption isotherm, respectively. This advantage stems from the fact that NO is a radical, offering the possibility to stabilize electrons as well as holes on the TiO2 surface. This results in the formation of NO/NO pairs. The proposed adsorption model, however, does not only consider this pair formation but, in addition, the adsorption of NO molecules on charged sites while the complementary charged sites are stabilized by traps present on the surface. The proposed adsorption isotherm is supported by experimental results.