The mechanism of photooxidation of sulfur dioxide in the presence of ozone and under irradiation at 266 nm has been investigated at low temperature in an argon matrix. Formation of stable SO3 occurs from the reaction between SO2 and O(D-1) and also by oxygen atom transfer within the SO2:O-3 complex. Use of isotopic SO2 species led to unusual observations suggesting that photooxidation of SO2 proceeds via an intermediate species, namely, OSOO; this trioxide sulfur isomer transforms into SO3 by tunneling. In the gas phase, the quenching of the O(D-1) atom by SO2 could occur through this intermediate.