Anodic oxide metal–insulator–semiconductor (MIS) structures have been prepared on (100)‐ and (111)B‐oriented samples ofn‐type InSb using either phosphoric or citric acid as the electrolyte. Interface properties have been evaluated using 1‐MHz capacitance–voltage (C–V) measurements as a function of temperature from ∼20 to 300 K. By comparing interface state density and fixed charge with preparation conditions for the dielectric, it is concluded that the data are consistent with the presence of elemental Sb at the interface, which acts as a hole trap.