A quantitative analysis of existing experimental data on the surface Fermi energies of vacuum- cleaved n- and p-type GaAs and InP as a function of oxygen exposure has been performed. A systematic approach has been developed for a two-level model with surface acceptor and donor energies Ea, Ed and densities Na,Nd to determine the admissible regions in the Ea,Ed plane. Two admissible energy regions are found for GaAs, but just one very restricted region for InP. With some reservations, the conventional assignments for Ea,Ed in GaAs and InP may be considered as compatible with these admissible regions. The derived dependence of Na,Nd on oxygen exposure, both for GaAs and InP, suggests the existence of two oxidation mechanisms, the first active at low surface exposures and saturating at 104–105 Langmuir, the second becoming dominant at 0.02– 0.05 monolayers of surface oxygen coverage and saturating at high-exposure levels.