Abstract: The SO32- in formaldehyde-stabilized solution is determined in a carrier of 0.1 M phosphate buffer, pH 8.5. The solution is passed through a column of immobilized sulfite oxidase (on controlled-pore glass) and the H2O2 liberated is measured amperometrically. The concentration. range is from 10 µM to 0.8 mM and the coefficient of variation at 0.4 mM is 2.2% (n = 3). Sulfite oxidase is determined in a stream of 2 mM Na2SO3 stabilized with formaldehyde in 0.1 M phosphate buffer, pH 8.0. The concentration. range is 5 to 100 iu l-1.