The thermally stimulated desorption (TSD) curves of oxygen (AMU 16) and cadmium from Class I photoconducting CdS platelets, grown in a N2H2S atmosphere, were studied in the temperature range from room temperature to 550°C. Substantial desorption of oxygen with a maximum around 350°C was observed. The TSD curve for cadmium exhibited a plateau near 400°C, attributed to the desorption of overstoichiometric cadmium from a thin layer near the crystal surface, followed by a sharp rise above 470°C associated with CdS evaporation. Changes in the spectral distribution of the photocurrent (SDP) and dark current were observed upon desorption. Their correlation with desorption and their reversibility upon exposure of the investigated platelets to O2 and an O2H2O mixture and to electron bombardment were investigated. A model for Class I crystals is presented, involving the existence of a Cd‐rich semiconducting layer near their surface, that explains the observed SDP and dark current behavior. In particular, the disappearance of such a layer upon desorption to around 400°C can account for the observed transition from Class I to Class II SDP.