A three-dimensional chemistry-transport model of the stratosphere is used to simulate the evolution of trace constituents during the 1991--1992 Arctic winter. It is shown that heterogeneous reactions on polar stratospheric clouds led in early January to almost complete activation of atmospheric chlorine inside the polar vortex, in remarkable coincidence with observations by the ER-2 aircraft (Toohey et al., 1993) and the microwave limb sounder on the Upper Atmosphere Research Satellite (Waters et al., 1993). Sulfate aerosols resulting from the eruption of Mount Pinatubo also produced a significant increase in chlorine monoxide (ClO) concentrations at middle and high latitudes. The net chemical destruction of ozone found in the vortex at the end of the simulation (25% at 50 hPa and 25 DU), although substantial, was limited by available sunlight and the short period during which stratospheric clouds occurred. ¿ American Geophysical Union 1994