Synchrotron radiation photoemission studies show that thin Yb overlayers (0.3–4.1) monolayers) enhance the oxidation of GaAs(110) surfaces. The magnitude of the promotion effect varies as a function of Yb coverage. The oxidation reaction products involve several nonequivalent oxidation states of As and Ga. The specific catalytic activity of the pure divalent Yb overlayers in promoting GaAs oxidation appears lower than that of Sm overlayers containing both Sm2+and Sm3+species. The spectroscopic signature of the oxidation reaction products, instead, is compellingly similar for the two rare‐earth promoters. We propose that the oxidation promotion mechanism is related, in both cases, to the decomposition of metal/semiconductor interface reaction products upon exposure to oxygen, and that the rare‐earth atomic valence has only a limited influence on the promotion mechanism.