A method of indirect polarographic determination of trichlorfou and dichlorvos was elaborated. Glyoxal, the product of alkaline hydrolysis of both these organophosphorus insecticides, condenses with o-phenylendiamine to forming quinoxaline, which is polarographically reduced, giving a well-developed two-electron wave. The relation of the concentration of the insecticides and corresponding diffusion current is recti linear in the broad concentration range of 0–8 mg. The standard deviation of this method is ± 1.2% for 400 jig and ± 5.2% for 3.0 μg of trichlorfon respectively.