The mechanism of the reactions of electronically excited SO2 with isobutane has been studied through the measurement of the initial quantum yields of product formation in 3130 A irradiated gaseous binary mixtures of SO2 and isobutane and ternary mixtures of S02, isobutane, C& or Con. Under low-pressure conditions ( P < 10 torr) the kinetic treatment of the present data shows that only one singlet and one triplet state, presumably the 'B1 and :3B1 states, are involved in the photoreaction mechanism. The data give kza = 8.4 X lo9; (2c); error limits are f 2 0. T t e contribution from the excited SOd'B1) molecules to the quantum yields of the photolyses of SOYisobutane mixtures is not negligible. Under high-pressure conditions ( P > 10 torr) the low-pressure mechanism coupled with the saturation effect on the phosphorescence lifetimes of S02(3B1) molecules cannot alone rationalize the quantum yields. The evaluation suggests that some nonradiative intermediate state (X) is involved in the formation of "extra" triplet molecules. This ill-defined state decays largely nonradiatively to so2 in experiments at low pressures, X -so2 (12). In the presence of CsHs the low-pressure data give k7 = (8.5 f 1.8) X lolo, and the high-pressure data give k7 = (8.3 * 0.6) X IO'O and (9.9 f 0.9) X lo'* L'molsec; SOz("1) + CsHs -nonradiative products (7).These estimates are in good agreement with values directly measured from low-pressure lifetime studies, (8.1 + 0.7) X 1O'O and (8.8 f 0.8) X 1O'O l./mol-sec.