On the basis of the electron theory of chemisorption, a criterion is deduced for the positive and negative photoadsorptive effect on semiconductors (photoadsorption and photodesorption). A relation is established between the sign of the effect and the nature of the adsorbent and adsorbate, the previous treatment of the sample, and the experimental conditions (temperature and pressure). The available experimental material is analyzed and interpreted from the standpoint of the proposed theory.