We examined the effect of ultrathin (0.1–10 Å) chromium overlayers on the reactivity with oxygen of Si(111) and GaAs(110) cleavage surfaces. Synchrotron radiation photoemission shows that for Cr coverages below a critical threshold coverage the overlayer does not affect substantially the oxygen adsorption rate. For chromium coverage above threshold the overlayer sharply enhances the oxygen adsorption kinetics so that most semiconductor atoms in the surface and near surface region appear oxidized at activated oxygen exposures as low as 100 L. The critical threshold coverage corresponds to the onset of reactive interdiffusion at the Si(111)–Cr and GaAs(110)–Cr interfaces. Therefore we suggest that ultrathin Si–Cr and As–Cr reacted phases created at the surface act as activation layers for semiconductor oxidation.