Differential capacitance-voltage (C-V) measurements were performed on Al and Pt rectifying contacts fabricated on natural (type IIb) diamonds. The C-V data showed frequency dependence, which decreased significantly after reducing the back contact impedance. The frequency dependence seems primarily to be an effect of the contact capacitance, contact resistance, and bulk resistance of diamond. A model which includes these variables has been proposed to explain this frequency dependence using both large and small back contact impedances.