Reflectance anisotropy spectroscopy (RAS) has been applied to study the oxidation of decapped InAs(0 0 1) surfaces at room temperature. (2 × 4) and (4 × 2) surfaces were prepared in ultra high vacuum by annealing, and then exposed to increasing amounts of molecular oxygen. During exposure to gas, RAS spectral structures at 2.4 eV in (2 × 4) and at 1.7 eV in (4 × 2) are progressively reduced. After about 1 × 104 L, a broad residual anisotropy spectrum is still measured for (2 × 4), while relic anisotropy is nearly null for (4 × 2). Residual features are visible at bulk critical points (E1, E1 + Δ1 and E′0). The different behavior versus gas contamination evidences the origin of RAS spectral structures, allowing interpretation in terms of surface-state or bulk transitions (the latter modified by the surface). By using Kramers-Kronig relations, the anisotropy of the imaginary part of the surface dielectric function (Δε″s) between [1̄ 1 0] and [1 1 0] directions of the substrate has been obtained from RAS data. We conclude that the main RAS structures (at 2.4 eV in (2 × 4), at 1.7 eV in (4 × 2)) are related to the surface reconstruction, although only the latter is a transition involving true surface states. We observe bulk contributions in coincidence with critical points (E1, E1 + Δ1 and E′0). © 2002 Elsevier Science B.V. All rights reserved.