GeO disproportionation into GeO2 and Ge is studied through x-ray photoelectron spectroscopy. Direct evidence for the reaction 2GeO → GeO2 + Ge after annealing in ultra-high vacuum is presented. Activation energy for GeO disproportionation is found to be about 0.7 ± 0.2 eV through kinetic and thermodynamic calculations. A kinetic model of GeO disproportionation is established by considering oxygen transfer in the GeO network. The relationship between GeO disproportionation and GeO desorption induced by GeO2/Ge interfacial reaction is discussed, and the apparent contradiction between GeO desorption via interfacial redox reaction and GeO disproportionation into Ge and GeO2 is explained by considering the oxygen vacancy.