A model calculation is made to investigate the effect of a quantizing magnetic field on the capacitance of MOS structures of small-gap semiconductors having n-channel inversion layers under the influence of strong surface electric fields. It is found, taking n-channel InSb as an example, that both the MOS and surface capacitances show spiky oscillations with changing magnetic field. This is in qualitative agreement with the experimental observations reported elsewhere in MOS structures of Si and Hg1−xCdxTe. It is further observed that the sharpness and the depths of the spikes increase with increasing magnetic field whereas the depths are found to decrease with increasing thickness of the insulating layer. PACS: 73, 85.30. © 1980 IOP Publishing Ltd.