We investigate the heretofore unstudied role of volcanic and nonvolcanic sulfur aerosols and gases in the mesosphere. Two-dimensional microphysical calculations show that sulfur may be an important source of particles below the cold summer mesopause. Observed increases in SO2 with altitude in the upper stratosphere had previously suggested ultraviolet destruction of H2SO4, discounting its survival in the mesosphere. Laboratory measurements have now ruled out ultraviolet photolysis of H2SO4, however, and a recent proposal of visible and near-infrared photolysis of H2SO4 explains the SO2 observations. Our calculations show that enough H2SO4 survives this weak photolysis mechanism to produce significant sulfate aerosol surface area in the mesosphere. Neutralization of H2SO4 by metals on the surfaces of meteoritic dust is modeled and affects the mesospheric aerosol negligibly. We discuss the possible implications for this new class of particles for the formation of polar mesospheric summer echoes and polar mesospheric clouds in volcanically quiescent and active periods.