Supercritical water desulfurization (SCWDS) has potential as a technique for removing sulfur from feedstocks such as heavy oil and bitumen. However, a fundamental understanding of SCWDS (such as the underlying chemical mechanisms, relative rates of desulfurization, and the role of SCW and hydrocarbons) is limited. In the present work, we have gained molecular-level insights into this process by measuring the kinetics of decomposition of a variety of organic sulfides in the presence of hydrocarbons and supercritical water in a continuously fed stirred-tank reactor (CSTR). The results are consistent with a free-radical mechanism, with hydrogen abstraction from the sulfide as the rate-determining step. The decomposition rates of the aliphatic and aromatic sulfides varied depending on their molecular structure, with conversions after 31 min at 400 degrees C ranging from less than 3% (our detection limits) to more than 90%. These differences in the reactivity correlate with the estimated heats of reaction for the critical hydrogen abstraction. The decomposition rates of the sulfides were affected by the presence of hydrocarbon carriers, with the rates being higher in the presence of alkanes than in the presence of toluene, as expected for a free-radical process. Product distributions and rates of radical-induced alkane cracking during this process were likewise affected by the presence of different sulfides. The decomposition of several different sulfides is consistent with 3/2 power kinetics, providing further evidence that the reaction proceeds via a radical mechanism. The knowledge developed in the current work provides a fundamental basis for further improvements in SCWDS.