We have constructed one-dimensional aerosol microphysical and photochemical models to examine the chemistry of stratospheric volcanic clouds. Estimates of the stratospheric inputs of several key volcanic gases were made. Our results suggest that the aerosol microphysical processes of condensation and coagulation produce larger particles as the SO2 injection rate is increased, rather than a larger number of particles of the same size. Larger particles have a smaller optical depth per unit mass than do smaller ones. They also settle out of the stratosphere at a faster rate, thereby restricting the total number of particles in the stratosphere. These processes moderate the impact of volcanic clouds on the Earth's radiation budget and climate and suggest that volcanic effects may be self-limiting. However, following large eruptions, the scattering of solar radiation by the aerosol and reaction with SO2 can deplete the abundance of OH radicals in the stratosphere. The injection of HCl into the stratosphere, which could lead to large ozone changes, is limited by a cold trap effect in which HCl and water vapor condense on ash particles in the rising volcanic plume and fall out as ice.