A potentiometric method for cholinesterase inhibitor analysis based on mediatorless bioelectrocatalysis has been developed. The method includes coimmobilization of three enzymes, butyrylcholinesterase, choline oxidase and peroxidase, on composite carbon electrodes. Catalytic hydrolysis of butyrylcholine and subsequent catalytic oxidation of choline result in the formation of hydrogen peroxide leads to a shift in the electrode potential. The detection limit for trichlorfon analysis is 2 x 10(-13) M. Electrodes remain stable for at least 4 weeks when stored at 277 K.