Volcanism is responsible for copious discharge of sulfur-bearing magmatic gases. A dominant sulfur-bearing species is SO2, yet sulfite (S4+) minerals are rare in geological settings, testifying to the reactivity of SO2. Disproportionation of SO2 to reduced (sulfide), neutral (elemental sulfur), and oxidized (sulfate) species is one expression of this reactivity. Previous studies of SO2 disproportionation reactions have focused on low-pressure, low-temperature conditions involving SO2 gas and liquid water, such as those close to volcanic vents or in water-free gas systems. However, release of magmatic volatiles in volcanic systems can occur at depth where hot magmatic gas meets crustal rocks ± pore fluids. We investigate the viability of such high-pressure reactions with experiments in which SO2 gas is reacted with a mixture of calcite and a metal-bearing saline fluid at 1.0–1.5 kbar pressure and 400–800 °C. At all temperatures, the calcite-bearing experiments produced anhydrite and sulfide. In the experiment where calcite was replaced by quartz, no sulfide was produced, establishing that Ca is a crucial component of this reaction. We show that extensive production of anhydrite and sulfide through calcite-mediated SO2 disproportionation takes place on time scales of just a few hours. This could occur where hot magmatic SO2 gas encounters magmatic brines in igneous rocks, basinal brines within sedimentary rocks, or calcium-bearing saline groundwaters.