The overlayers of Cs2O, Cs2O2 and CsO2/Cs2O2 were prepared by simultaneous oxygen-cesium adsorption on Ge(1 1 1) at room temperature. Electron spectroscopy has been used to investigate the surfaces of Cs2O/Ge, Cs2O2/Ge and CsO2/Cs2O2/Ge as a function of annealing temperature. The interaction of Cs2O with Ge(1 1 1) is weak and the surface has a work-function value of 0.8-0.9 eV before the decomposition of Cs2O at 660 K. The work function of the Cs2O2/Ge surface is 1.3 eV and decreases to a minimum value of 0.95 eV after annealing at 440 K. Both Cs-O and Ge-O bonds are formed before the Cs2O2 species decomposes at 550 K. The results show that the CsO2/Cs2O2/Ge system is reactive as some Ge atoms are bonded to oxygen after coadsorption. The work function of the surface remains 2.15 eV before the decomposition of CsO2. For all surfaces, when the cesium oxides decompose, both Cs-O and Ge-O bonds are dominant with a work function of about 1.2 eV. Further annealing gives rise to the increases of the work function.