To predict the performance of SOI transistors, it is essential that material dimensions and parameters are accurately known. It is demonstrated here that from a single high frequency capacitance-voltage (C-V) plot performed on an SOI capacitor, one can obtain the buried insulator and body (active layer) thickness. The results obtained from the method are verified by independent C-V measurements carried out on the same device. In the present case this technique is demonstrated using SIMOX material but applies equally well to other SOI technologies. Because of the ease of fabrication of the capacitor such a measurement can be performed on starting material. The measurement strictly applies to the condition of a fully depleted body, but in the partially depleted case the thickness of the buried oxide can still be estimated.