We have investigated the wet anodization of InSb under a large variety of oxidation parameters. Results of Auger electron spectroscopy (AES) show that the oxidation produces a mixed oxide film of In2O3 and Sb2O3 containing some unoxidized Sb. 1-MHz capacitance-voltage characteristics show low-frequency-like response of the oxide-semiconductor junction in the inversion region. We have found a strong correlation between this response and the elemental Sb content in the oxide, as revealed by AES. We present a model which explains the role of the elemental Sb in terms of interface traps and charge transfer into the inversion layer. Furthermore, we describe the anodization conditions under which the combined effect is reduced and explain their role. The implications of this observation are discussed.