Oxide layers on undoped GaAs substrates have been assessed by grazing incidence X-ray reflectivity (GIXR), spectroscopic ellipsometry (SE), surface photoabsorption (SPA) and atomic force microscopy (AFM). In addition to providing a comparison between different measurement techniques, these new data improve the understanding of the structure and thermal desorption of oxides typical to GaAs substrates. Epi-ready GaAs wafers typically have an estimated oxide layer thickness of between 23-30 Angstrom and exhibit a surface roughness of 2-3 Angstrom. Furthermore, a significant change in the oxide chemical species through the film is indicated. An activation energy for desorption of Ga2O3 of 2.0 eV is calculated, whilst partial deoxidation of acid-polished GaAs suggests oxide removal proceeds as island shrinkage of successive oxide layers.