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.