Chemical changes of dissolved SO₂ were followed in vitro system. Varying concentrations of sulfite solution reached similar Eh values 20 hours after preparation. No further increase of Eh values occurred in acidic solutions. Eh values were more greatly increased in neutral solutions than in alkaline solutions, and slightly higher in light than in dark. The decrease of measurable SO₂ content tended to be greater with increasing concentrations of sulfite, on exposure to light, and at higher pH values. A larger amount of hydrogen atoms was produced with higher concentrations of SO₂ and in neutral to alkaline solutions than in acidic solutions.