The flatband potential of silicon in fluoride solutions with different pH values has been measured by evaluation of the potential-modulation-induced microwave reflectivity. The theory of potential-modulation-induced microwave reflectivity measurements is outlined and the method is contrasted with conventional admittance measurements. The dependence of the flatband potential on the pH of the solution was found to be Nernstian over a wide range of pH. Comparison of the microwave and admittance responses allowed the capacitance of surface states to be determined as a function of potential for different solutions.