The effective second-order rate constant for O + SO 2 (+Ar) fi 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 290-840 K. The results obtained at pressures up to 880 mbar 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 À23 T À3 exp (À2400/T) cm 6 molecule À2 s À1 and the high-pressure limit k 1 = 6.1 · 10 À13 exp (À850/T) cm 3 molecule À1 s À1. An RRKM extrapolation of the present k 0 values yields k 0 = 6.7 · 10 À21 T À3.6 exp (À2610/T) cm 6 molecule À2 s À1 , which is in good accord with several literature values over 220-2500 K. At low temperatures, k 0 shows a positive activation energy rationalized in terms of a barrier of 15.9 kJ mol À1, and k 0 reaches a maximum at around 750 K. Our expression for k 1 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.