The effect of an ammonium sulfide treatment on the GaAs (100) surface has been investigated by x‐ray photoelectron spectroscopy. The treatment produces a slight Ga enrichment on the surface and leaves roughly 0.6 of a monolayer of sulfide which inhibits initial oxidation of the surface. The sulfide is not lost as the surface becomes oxidized but appears to remain near the GaAs/oxide interface. Furthermore, in the oxidized layer, As oxide is preferentially drawn to the surface relative to Ga oxide.