The adsorption of insulin and soybean trypsin inhibitor (STI) on the mercury electrode was investigated with phase-sensitive alternating current (ac) polarography. The investigations focused on the effect of insulin and STI concentrations on the capacitive ac component over a wide potential range at pH 8.8, when both proteins are in folded form, and at pH 1.0 when they are unfolded. Besides a pseudo-capacitance due to the faradaic process of protein -S-S- bond reduction, the presence of protein adsorption of the Frumkin type was established. From the analysis of the corresponding adsorption isotherms, the relevant thermodynamic adsorption parameters were determined. Similarities in the adsorption characteristics of both proteins studied were demonstrated, and the differences in the adsorption behaviour between native and denatured proteins were proven. The data obtained is also discussed from the aspect of protein disulphide bond interaction with mercury.