Negative‐electron‐affinity GaAs photocathodes have been improved by optimizing the (Cs,O)/GaAs interface. An optimum interface can be obtained by annealing the preoxygenated substrate surface at elevated temperatures (∼450 °C) prior to the (Cs,O) activation. Furthermore, the results of experiments with the aid of x‐ray photoelectron spectroscopy demonstrate that the direct bonding of oxygen to substrate atoms is most likely the origin of the interfacial barrier to electron escape.