We have studied the oxidation of Rb/Si(100), Cs/InP(110), and Cs/GaP(110) surfaces by core-level photoelectron spectroscopy, monitoring the growth of oxide as a function of alkali-metal coverage and oxygen exposure. The oxidation of the substrate is in all three cases strongly enhanced by the presence of alkali-metals. For GaP(110), this is the first observation of such effects. For Si(100), with a given Rb coverage, the amount of oxide depends linearly on the logarithmic oxygen exposure. This is explained in terms of a non-local model based on dissociative sticking of O 2, which in turn is facilitated by electron transfer into antibonding molecular orbitals. The slow increase of the work function upon oxidation is of crucial importance in this model. The model can be generalized to other alkali-covered Si and Ge surfaces. The oxidation kinetics of caesiated InP and GaP surfaces is more complicated, and will require better knowledge about the interface structure to be properly understood. Our measured In4d spectra, however, contradict recent suggestions that indium metal segregates from the Cs/InP(110) interface.