Abstract In the present investigation 0.01 M aqueous oxaldihydroxamic acid has been used to stabilize the atmospheric sulfur dioxide. The collection efficiency of the reagent was found to be ~ 100% and the sulfite solution was stable for ⩾ 30 days at room temperature. The sulfite ion was estimated colorimetrically using acidified p-aminoazobenzene and formaldehyde. The pink coloured dye, λ max 505 nm, obeys Beer's law in the range of 0.1–1 ppm. The procedure has been optimized with respect to the acidity, time and reagent concentration. The method is simple, free from pH dependence and several commonly present air pollutants do not interfere.