The importance of chemical processes on polar stratospheric clouds (PSCs) has been investigated in the context of the reported ozone depletion in middle and high northern latitudes. Using a variety of PSC frequencies during the polar winter and early spring the observed ozone depletion can be reproduced in a two-dimensional model of the atmosphere. Support for the mechanism is provided by the behavior of other model variables. For example, a ClO mixing ratio of more than 600 pptv is predicted within the polar vortex; the ClONO2 column peaks around the edge, or ''collar'', of the vortex. These and other results resemble the findings of recent Antarctic expeditions. In runs with low levels of chlorine, the presence of PSCs leads to ozone production due to the inhibition of the classical NOx catalytic destruction cycle. This could have implications for strategies to control chlorine-containing compounds. ¿ American Geophysical Union 1990