A potentiometric investigation of solid-state enzyme electrodes prepared by electropolymerization of hydrolytic enzymes (acetylcholinesterase, urease and arginase) on the surface of inert electrodes is reported . The theoretical description of the membrane potential is based on the Donnan effect and diffusion potential. It is shown that the ionic strength, pH and nature of the electrolyte influence the magnitude and sign of the membrane potential . At high pH and at high ionic strength the membrane potential is mainly a function of the diffusion potential . The mechanism of direct transformation of enzymatic reactions into an electric signal is considered in detail with the example of an acetylcholinesterase sensor. The solid-state enzyme electrode was tested in the determination of acetylcholine, urea and arginine. Typical calibration graphs for these substrates showed a linear response over the range 10 -n-10 -3 M. The solid-state enzyme electrodes presented here do not require the specific introduction of an internal reference system.