A new and simple spectrophotometric method for the determination of phorate, also known as ‘Thimet’, a widely used systemic insecticide is proposed. The method is based on the oxidation of phorate to sulphones by concentrated sulphuric acid and its hydrolysis under acidic conditions at ∼100°C to release formaldehyde which is then treated with J-acid to produce a yellow fluorescent color, which has an absorbance maximum at 470 nm. Beer's law is obeyed in the range of 1–8 ppm of phorate. The method has been applied to the determination of phorate in effluent water, rice, wheat and cabbage samples. The method is free from interferences of most of the commonly used pesticides and other common ions.