Four different methods for calculating interface state density in vertical SiO2/4H-SiC metal-oxide semiconductor (MOS) capacitors were employed on the same samples. The Terman, ac, quasi-static, and Hi-Lo methods were used to extract surface state densities from the SiO2/SiC interface. Surface state densities from 10(11) to 10(12) cm(-2) eV(-1) were obtained, depending on the method employed. The Hi-Lo method is particularly susceptible to underestimating the trap density if UV light is not used to empty all the deep-lying traps during measurement. (C) 2003 The Electrochemical Society.