Isoprene can react with sulfoxy radicals (SO4•- and SO3•-) to form organosulfur compounds in aqueous phase, and the organosulfur compounds are important compositions of secondary organic aerosols (SOAs). To make sure the specific configurations of the products and the role of SO4•- and SO3•- in the formation of organosulfur compounds, the reaction mechanisms are studied by theoretical calculations. The lowest Gibbs free energy barrier of addition of SO3•- onto isoprene is 24.06 kcal mol-1 at C4 site, and its rate constant is 1.30 × 10-11 M-1 s-1 at 298 K and 1 atm. And the Gibbs free energy barriers of addition of SO4•- onto isoprene at C1 and C4 sites are barrierless and 0.92 kcal mol-1; the rate constants of these two addition processes are 6.85 × 109 and 1.17 × 105 M-1 s-1 at 298 K and 1 atm. It elucidates that organosulfates are easier to be formed. As for the products P1 (with alcohol group) and P2 (with aldehyde group), the lowest Gibbs free energy barrier of the formation of P1 is 3.17 kcal mol-1, and that of the formation of P2 is 15.84 kcal mol-1, which means that the product with alcohol group is easier to be formed than that with aldehyde group. This work provides a reference for the formation of organosulfur compounds in aqueous phase, and it may help to understand the SOA formation.