Negative‐electron‐affinity (NEA) photocathodes are made possible by adding Cs and oxygen to the surface of Si or GaAs and other III‐V materials. In this work, the chemistry of the Cs‐O adlayer on GaAs is studied by following the Cs, Ga, and As core‐level shifts ath&ngr;=120 eV. The changes in binding energy for the Cs core levels were studied as a function of oxygen exposure. Oxygen was found to bind to As of GaAs for exposures above 10 L. These results are discussed in terms of the importance of Cs suboxides, in relation to the interfacial barriers which limit the ultimate photothreshold of NEA cathodes.