An attempt is made to formulate the gate capacitance of MOS structures of ternary semiconductors having n-channel inversion layers under magnetic quantization without any approximations of weak or strong electric field limits. It is found, taking n-channel Hg1-xCdxTe as an example, that the gate capacitance exhibits spiky oscillations with the reciprocal magnetic field. It is further observed that the sharpness and the amplitudes of spikes increase with increasing magnetic field. Besides, the corresponding results for n-channel inversion layers on parabolic semiconductors are also obtained from the expressions derived. © 1985 IOP Publishing Ltd.