A Cs2O/Cs2O2 overlayer was prepared by simultaneous oxygen-cesium adsorption on GaAs(I 10) at room temperature. In situ electron spectroscopy and work function measurements have been used to study the Cs2O/Cs2O2/GaAs surface as a function of annealing temperature. The Cs2O/Cs2O2/GaAs surface exhibits metallic and has a work function value of 0.9 eV. There are Cs-O-Ga, Cs-O-As, As-O, and Ga-O bonds at the interface of Cs2O2/GaAs. Annealing facilitates the interfacial reaction and the minimum of the work function (0.7 eV) was observed after annealing at 540 K. After the decomposition Of Cs2O and Cs2O2 at 620 K, the surface dominated by the Cs-O-Ga and Cs-O-As species remains negative electron affinity with a work function value of 1.1 eV. For further annealing, the fast Cs desorption is associated with the decomposition of the Cs-O-Ga and Ca-O-As bonds as well as the evaporations of As2O3 and CsAsO2, forming Ga2O3.