The elementary reaction SO2 + CO --> CO2 + SO((3)Sigma) (1) and the subsequent reaction SO((3)Sigma) + CO --> CO2 + S(P-3) (2) have been studied by the application of the Gaussian-3//B3LYP quantum chemical approach to characterize the potential energy surfaces and transition state kinetic analysis to derive rate coefficients. Reaction I is found to take place via two transition states (TS), a cis-OSOCO TS and a trans-OSOCO TS. Reaction via the cis-TS is concerted and takes place on a singlet surface. Intersystem crossing to the final products occurs after passage through the barrier on the singlet surface. The trans-TS leads to a very weakly bound singlet OSOCO intermediate that then passes through a second TS (on the triplet surface) to form the products. Reaction 2 takes place on triplet surfaces. There is a concerted reaction through a cis-SOCO TS and a weakly bound trans-SOCO has also been identified. Reaction 2 is analogous to the reaction CO + O-2 ((3)Sigma) --> CO2 + O(P-3) (3), and this reaction has been reinvestigated at a similar level of theory and the rate coefficient derived by quantum chemistry is compared with experiment. The sensitive effects of trace impurities such as H-2, H2O, and hydrocarbons on the accurate experimental determination of the rate coefficient of reaction 3 is discussed. Using rate coefficients for reactions 1 and 2 obtained via quantum chemical calculations, we have been unable to model the extent of decomposition of SO2 measured in a shock tube study of reaction between SO2 and CO [Bauer, S. H.; Jeffers, P.; Lifshitz, A.; Yadava, B. P. Proc. Combust. Inst. 1971, 13, 417]. In light of the known sensitivity of reaction 3 to trace impurities, we have incorporated trace amounts of H-2, CH4, or H2O, together with our rate coefficients for (1) and (2), in a kinetic model of Alzueta et al. [Combust. Flame 2001, 127, 2234], which is then shown to be able to substantially model the SO2 data of Bauer et al. In the course of this modeling study we also computed heats of formation for a number of sulfur-containing small molecules: HS, HSO, HSOH, HOSO, HS2, HSO2, HOSO2, HOSOH, and HOSHO.