Photoemission　stability　of　the　negative　electron　affinity　(NEA)　GaAs-(Cs,O)　photocathode　activated　with　several　cycles　of　alternate　Cs　and　O2　adsorption　at　room　temperature　is　mainly　determined　by　the　stabilized　of　the　cesium　and　oxygen　adlayer　on　the　GaAs　surface,　and　the　bonding　strength　between　this　adlayer　and　the　GaAs　substrate.　It　is　found　that　the　light　treatment　(incident　white　light　illumination)　during　or　after　the　Cs　and　02　activation　process　can　improve　the　emission　stability　of　the　NEA　GaAs-(Cs,O)　cathodes.　It　is　also　found　that　the　emission　stability　of　the　NEA　GaAs-(Cs,O)　cathodes　is　closely　related　to　the　electronic　states　of　cesium　and　oxygen　before　being　adsorbed　on　the　GaAs　surfaces,　best　result　being　obtained　with　cesium　ions　and　excited　oxygen　molecules.　In　addition　to　the　activation　procedure　and　experimental　results,　some　photoinduced　reactions,　e.g.,　photoinduced　adsorption,　desorption,　excitation,　dissociation,　ionization,　and　surface　migration　and　surface　rearrangement　of　the　cesium　and　oxygen　adsorbates　on　or　from　GaAs　surfaces　are　analyzed,　and　their　effects　on　the　emission　stability　of　the　NEA　GaAs-(Cs,O)　photocathodes　are　discussed.　©　1993　SPIE.　All　rights　reserved.