Abstract A method for determining low levels of sulfite in water is described based on the liberation of sulfur dioxide from an acidified water sample by purging with nitrogen gas and re-absorbing it in a solution containing ferric ion and 1,10-phenanthroliné. The ferric ion is reduced to ferrous ion by the sulfur dioxide and the orange Tris-(1,10-phenanthroline)iron(II) complex is formed. After removing the excess ferric ion with ammonium bifluoride to stop further color development, the stable complex is measured colorimetrically at 510 nm. The detection limit is 0.01 mg l −1 SO 2− 3. The effect of and removal of interferences and the effect of temperature are discussed.