AbstractThe mechanism of the electronically excited sulfur‐dioxide sensitized isomerization ofcis‐2‐butene has been studied through the measurement of the initial quantum yields oftrans‐2‐butene formation in 3130‐A irradiated gaseous binary mixtures of SO2andcis‐2‐butene and ternary mixtures of SO2,cis‐2‐butene, and CO2. The kinetic treatment of the present data from the SO2C4H8mixtures and those of recent similar studies of Penzhorn and Güsten [3] and Cox [4]are all consistent with the involvement of only the long‐lived fluorescent1B1and phosphorescent3B1states of SO2in the isomerization mechanism. The data give (k2a+k2b)=0.21±0.04; SO2(1B1) + SO2→ SO2(3B1) + SO2(2a); SO2(1B1) + SO2→ (2SO2) (2b). The analogous intersystem crossing ratio for the SO2(1B1)‐cis‐2‐butene quenching collisions is the largest observed among the many quencher molecules studied; the value lies in the range of 0.85 to 0.37, with the extremes representing different choices of alternative data and possible mechanisms. From the present data the ratio of the SO2(3B1) quenching rate constant with SO2to that withcis‐2‐butene as quencher is estimated to be (2.7±1.2) × 10−3, in good agreement with our directly measured ratio from lifetime studies (2.91±0.23) × 10−3[30−32], and the value found in isomerization studies by Cox (2.40±0.09) × 10−3[4]. The simple two‐excited state mechanism, which is seemingly applicable to the relatively low‐pressure binary SO2‐butene mixture results, is not adequate to explain the data obtained in experiments with large quantities of added CO2gas. Here an “excess” quantum yield of isomerization is observed. Several alternative mechanisms can be used to rationalize these results, but all alternatives must incorporate some other excited SO2species (X) as well as SO2(3B1) and SO2(1B1). The kinetics suggest that the ill‐defined state is unreactive toward the olefin and decays nonradiatively to SO2largely in experiments at the lower pressures, X → SO2(11); it may generate SO2(3B1) in a collisionally induced process at high added inert gas (CO2) pressures, X + CO2→ SO2(3B1) + CO2(10a)