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An electrical impedance based specific binding assay which utilizes the avidin-biotin interaction is described. The specific binding of a biotinylated enzyme label, alkaline phosphatase, is determined by monitoring the change in capacitance of an avidin-coated planar gold electrode. The enzyme label alkaline phosphdtase converts the substrate 5-bromo-4-chloro-3-indolyl phosphate to an insoluble dimeric product which adheres specifically to the surface of the gold electrode and causes a maximum decrease in electrode capacitance of 55%. This capacitance change is directly related to the amount of biotinylated enzyme bound to the surface of the electrode. Electrical impedance analysis has been used to characterize the surface effects on the electrode.