A solid-state gas sensor for sulfur dioxide with a new working mechanism was fabricated and its sensing properties investigated. The sensor was constructed using yttria-stabilized zirconia as the oxygen ion conductor, silver sulfate based salts as the auxiliary phase, a silver sensing electrode and a platinum reference electrode. When sulfur dioxide was detected, the potential of the sensing electrode changed to negative with the new working mechanism. The 90% response time of the sensor to 100 ppm SO2 was ca. 40 s at 600 degrees C, and the electromotive force (EMF) value obtained was perfectly Linear to the logarithm of SO2 concentration over a wide range (1 to 1000 ppm). The Nernst's slope corresponded exactly to the theoretical value based on a two-electron electrochemical reaction.