Fatigue of AgCs2O, AgCs surfaces has been observed in blue and red light. In blue light the long wave threshold shifts to shorter wave‐length and the long wave maximum increases and sometimes shifts to shorter wave‐length. Rise of temperature at the cathode counteracts the phenomena. In red light temperature effects are dominating. The observed phenomena are in line with de Boer's theory of the photoelectric effect on such surfaces as the formation of positive ions on the surface causes polarization of the neighboring Cs‐atoms and a change in their selective absorption.