The results of a comprehensive study of the overall electrical characteristics of thermally oxidised silicon surfaces are presented, and interpreted on the basis of a simple physical model of the MOS structure. Extreme care was taken throughout this study to insure the validity, significance and reproducibility of the quantities measured. It is shown that the cHRGe in the surface states is constant over a wide range of variation of the surface potential, and that this cHRGe is positive for both n and p-type oxidised silicon. The density of surface state cHRGe is about 2 x 10 to the eleventh power CM-2, and is essentially unaffected by a twenty-fold variation in the oxide thickness and by a 250-fold variation in the conce2ntration of boron in the p-type samples. The effects of measurement frequency, illumination and temperature on the space cHRGe capacitance of an inverted surface were investigated. This capacitance was shown to follow one of three simple models depending on the measurement frequency and the rate of generation of minority carriers.