The effective second-order rate constant for O+SO 2 (+Ar)→SO 3 (+Ar) has been measured by pulsed laser-photolysis with resonance fluorescence detection of atomic oxygen under pseudo-first-order conditions. The pressure dependence was determined at five temperatures in the range 290–840K. The results obtained at pressures up to 880mbar reveal fall-off behavior. With an estimated broadening parameter F c , Troe fits to the measurements yield the low-pressure limit k 0 =9.5×10 −23 T −3 exp(−2400/T)cm 6 molecule −2 s −1 and the high-pressure limit k ∞ =6.1×10 −13 exp(−850/T)cm 3 molecule −1 s −1 . An RRKM extrapolation of the present k 0 values yields k 0 =6.7×10 −21 T −3.6 exp(−2610/T)cm 6 molecule −2 s −1 , which is in good accord with several literature values over 220–2500K. At low temperatures, k 0 shows a positive activation energy rationalized in terms of a barrier of 15.9kJmol −1 , and k 0 reaches a maximum at around 750K. Our expression for k ∞ is in order-of-magnitude accord with values used to model a flow reactor and indicates a small pre-exponential factor. This may reflect the probability of triplet–singlet transitions in the spin-forbidden addition reaction and/or a tight transition state.