Enzyme electrodes for the determination of organophosphate pesticides were developed by using acetylcholinesterase (AChE) in combination with a pH electrode. AChE was immobilized on the surface of pH electrode by using gelatin and chitosan membranes, respectively. The measurement system is based on potentiometric detection of the inhibiting properties of organophosphates on enzyme activity. The phosphate buffer (2.5mM, pH 8.0) and the borate buffer (2.5mM, pH 8.5) at 25 degrees C were established as providing the optimum conditions for the gelatin and chitosan based biosensor systems, respectively. Furthermore, operational, thermal, and organic phase stabilities were also tested. Linear ranges for different organophosphates such as malathion, parathion-methyl, and methamidophos were detected by using both types of biosensor system. Moreover, as well as accuracy, the regeneration conditions by using pyridine-2-aldoximethiodide (PAM-2) as a reactivating agent for the inhibited electrodes were also investigated.