An electrochemical method to selectively determine sulfur dioxide in the presence of sulfur trioxide was investigated. A hydrophobic gas- permeable membrane separates the gas sample solution from an internal filling solution containing a fixed quantity of chloride ion and a sufficiently large excess of sodium bisulfite so that its concentration may be considered constant. Sulfur dioxide diffuses through the membrane to dissolve in the internal filling solution and liberates an equivalent quantity of hydrogen ions. The electromotive force between a precision-ground, flat-ended ph electrode, which acts as the sensing element, and a chloride sensing electrode, which acts as the reference element, gives a direct measure of so2 in the gas sample solution. Calibration curves in water, tetrachloromercurate (tcm), and citrate buffer absorption solutions were closely parallel with slopes in their linear segments, ranging from 57 to 52 mv/decade of so2 concentration. The slopes compare reasonably well with the theoretically expected slope of 58 millivolts at 21 deg c. The water standard curve was linear with respect to the logarithm of so2 concentration from 1 to 1,000 ppm so2; the tcm standard curve was linear from 10 to 2,000 ppm so2; and the citrate standard curve was linear from 30 to 5,000 ppm so2. The high efficiency and nontoxicity of citrate favor it as a substitute for the conventionally used tcm in the absorption solution.