Abstract A new spectrophotometric determination of parathion-methyl using guaiacol as a new reagent is described. The method is based on the reduction of the nitro group present in parathion-methyl with zinc/HCl to form amino-parathion-methyl. The aminoparathion-methyl thus formed is diazotized and subsequently coupled with guaiacol to form yellow-colored azo dye in alkaline medium. The dye shows an absorbance maxima at 470 nm and obeys Beer's law in the range of 15–200 μg parathion-methyl in 25 ml final solution (0.6–8 ppm). The important analytical parameters and optimum reaction conditions have been evaluated. The method has been successfully applied to the determination of parathion-methyl in soil and various vegetables.