Colorimetric sensing membranes for the determination of sulfur dioxide were developed and characterized. These films can be used for sensing trace amounts of sulfur dioxide both in the gas phase and in aqueous solutions. Lipophilic pH indicator ion pairs were immobilized in hydrophobic gas-permeable silicone and phenyl substituted ormosil. On exposure to SO₂ the films undergo a visually detectable color change from blue to yellow. No cross-sensitivity to pH and CO₂ was observed. Response times depend on the thickness of the sensing membranes, the indicator concentration in the film as well as on the respective SO₂ concentration. Membranes with response times of < 1 min (t₉o) were developed. The sensitivity to sulfur dioxide depends on the pKa of the indicator. An increase in the pKa results in a lower detection limit. The new optical SO₂ sensors are chemically and mechanically stable and are easy to manufacture. The storage stability of the membranes is at least 7 months if stored in the dark.