Processes and systems associated with hydrodynamic cavitation-catalyzed oxidation of sulfur-containing substances in a fluid are described. In one example method, carbonaceous fluid is combined with at least one oxidant to form a mixture and then the mixture is flowed through at least one local constriction in a flow-through chamber at a sufficient pressure and flow rate to create hydrodynamic cavitation in the flowing mixture having a power density of between about 3,600 kWatts/cm2 and about 56,000 kWatts/cm2 measured at the surface of the local constriction normal to the direction of fluid flow. The creation of hydrodynamic cavitation in the flowing mixture initiates one or more chemical reactions that, at least in part, oxidize at least some of the sulfur-containing substances in the carbonaceous fluid. An example system includes a device configured to mix a carbonaceous fluid and one or more oxidants, at least one cavitation chamber configured to produce cavitation bubbles in the mixture, and at least one
