The value and sign of a photo-emf appearing under optical excitation of the interface region between doped and undoped As 2 S 3 are measured. This emf is connected with the generation of an additional electric field, the magnitude and direction of which promote transfer of Ag + ions into undoped As 2 S 3 during the photodoping process. This result, combined with data obtained earlier about properties of the doped As 2 S 3 layer, the width of the interface layer between doped and undoped As 2 S 3 , and about the band structure of this region, enabled a new photoelectric model of the photodoping phenomenon to be suggested. This model can explain the main features of photodoping: the kinetics of Ag dissolution, the step-like profile of Ag concentration, the temperature and spectral dependences of light-sensitivity of the As 2 S 3 Ag structure.