SO2/O2 mixtures were photolyzed at 3130 Å and in the range 2500–4000 Å at room temperature. The only product of photolysis was SO3. Attempts to estimate ф(S03) using mass spectrometry, l.R. spectroscopy and pressure change measurements were unsuccessful, because it was not possible to obtain reproducible quantitative estimates of SO3. ф(SO2) values were determined by monitoring the 3130 Å absorption for its concentration measurements. ф(SO2) was independent of SO2 (11.6 to 50.4 torr) and O2 (50.0 to 390.6 torr) pressures. At 3130 Å, ф(SO2) varied between 1.5 × 10−2 and 2.2 X 10−2. Over the integrated range 2500–4000 Å ф(SO2) values of 2.1 X 10−3 to 2.9 X 10−3 were obtained. The differences in ф(SO2) values are explained in terms of wavelength dependence of the rate constants for the two primary reactions: 1SO2 + SO2 → 2SO2(1) and 1SO2 + SO2 → 3SO2 + SO2(2); (k2/k1) 3130 Å ≈ 10(k2/k1)2500–4000 Å.