The influence of HF (5%) on the surface of GaAs (100) and the oxidation of HF treated samples in laboratory air have been investigated by x-ray photoelectron spectroscopy, angle-resolved monochromatized photoelectron spectroscopy, and time-of-flight secondary ion mass spectroscopy. Etching with HF does not only remove the oxide but leads to an increasing loss of Ga from the surface. After some minutes a layer of elemental As is formed which grows linearly with the logarithm of HF storage time. Subsequent oxidation starts with the formation of As2O3 and Ga2O3 with preferential oxidation of As. After some hours, higher oxidized states of As and Ga [As2O5, GaAsO4, Ga(OH)3] are formed under the influence of light. The rate of As oxidation decreases until a state is reached which consists of nearly equal amounts of As and Ga. For storage times less than one week, the oxide thickness increases linearly with the logarithm of storage time, which is consistent with the Cabrera-Mott mechanism found earlier for the oxidation of GaAs.