Abstract: A modification of replacing the hazardous and corrosive conc.sulfuric acid medium in the photometric determination of formaldehyde with chromotropic acid by a medium of a mixture containning conc.phosphoric acid and hydrogen peroxide was proposed.In this conc.phosphoric acid medium,a violet-red compound was formed between chromotropic acid and formaldehyde which is photometrically determined at 570 nm.Beer′s Law is obeyed in the concentration range of 0.8-4.8 mg of formaldehyde per liter,with a correlation coefficient of 0.993 3.Recoveries obtained were in the range of 92.7%-105.3%.