In order to investigate the dramatic enhancement of the oxidation rate of semiconductor materials, promoted by an alkali-metal overlayer, we exposed a GaAs(110) surface covered by a monolayer of Cs to oxygen, observing the photoemission spectra of As 2p, Ga 2p, Cs 4d, and O Is. We show evidence that the oxidation mechanism proceeds through (i) oxygen infiltration underneath the Cs monolayer; (ii) disruption of low-binding-energy bonds Cs-As and Cs-Ga established at the Cs/GaAs(110) interface before the exposure to O-2; and (iii) oxide formation upon oxygen exposure, involving not only As and Ga but also Cs: substoichiometric and stoichiometric arsenic oxides are evident in different high-binding-energy components in the As 2p spectra. Ga oxide and Cs hybrid bonds were deduced by deconvolution of Ga 2p and Cs 4d core-level spectra. We show that the catalytic mechanism of the oxide promotion is due to Cs bound to the semiconducting elements through the formation of a positively charged layer which strongly attracts oxygen; afterwards, at the Cs surface, oxygen is preferentially driven toward the most stable bonds, i.e., the highest-binding-energy oxide components.