The capacitance-voltage characteristics of an anisotype heterojunction have been studied considering the presence of interface states and series resistance. The dependence of the above characteristics on the interface state density, doping concentration, temperature and series resistance has been evaluated, It is shown that the functional dependence of the device capacitance is generally determined by the surface potentials on the two sides of the junction. The value of the diffusion potential obtained under the limiting case of low interface state density and series resistance has been found about 0.1 V less compared to the experimental result of Unlu et al. [Appl, Phys, Lett. 56 (1990) 842]. This discrepancy can possibly be attributed to the effect of frequency on the device capacitance,