Organophosphorus pesticides (OPPs), diazinon (0.01-0.05 ppm) and ethylthiometon (0.01-0.2 ppm), were determined by monitoring the activity change of acetylcholinesterase (AChE), based on the chemisorption/desorption-process of thiocholine on a silver electrode as follows. Thiocholine, which was produced through the AChE-catalyzed reaction, was chemisorbed on the silver electrode surface, and then was cathodically desorbed. The electrical charge passed through the desorption process was proportional to the number of adsorbed thiocholine molecules, hence to the AChE activity. Thus the inhibition of the AChE activity by OPPs could be monitored from the decrease in the electrical charge for the desorption process.