A new technique of in situ capacitance-voltage (C-V) measurements is proposed to characterize the anisotropic etching of silicon in a two-electrode electrochemical cell. A series of C-V measurements have been performed on n- and p-type silicon,as the voltage is swept from etching to the passivation regime in the electrochemical cell. It is demonstrated that metal-oxide-semiconductor C-V analysis can be applied to identify the silicon etching and the passivation regions. The passivation oxide thickness has been extracted from the C-V curves over the range of the voltage used in the experiments. The values of the oxide thickness obtained from the C-V measurements are compared with those obtained from ellipsometric and spectroscopic measurements, and the discrepancies are explained on the basis of a varying dielectric constant value of the passivation oxide. For a passivation oxide film thickness greater than 300 Angstrom, the dielectric constant approaches 6, whereas for thinner films, a much higher value is obtained. It has been shown that the growth rate of passivation oxide is much higher for p-silicon compared to n-silicon. The passivation potential has also been inferred from the C-V curves.