The influence of illumination intensity and residual gases on the stability of negative electron affinity GaAs(Cs,O) photocathodes during operation was investigated in the present work. A comparison was made between the stability of photocathodes installed in activation chamber and tube body. Studies of the activated GaAs photocathode surface and the surface of GaAs photocathode with sensitivity decaying to zero were made using Auger electron spectroscopy. It was found that the degradation of GaAs photocathodes stems maingly from the interaction of harmful residual gases and photocathode surface.