Satellite observations of cloud optical depths and occurrence frequencies are used as input to a two-dimensional numerical model of the chemistry and dynamics of the atmosphere to study the effects of heterogeneous reactions on cloud surfaces upon chemical composition and ozone depletion in the tropopause region. Efficient reactions of CIONO2 with HCl and H2O, and of HOCl with HCl, are likely to take place on the surfaces of cirrus clouds <Borrmann et al., 1996> and perturb chlorine chemistry, much as they do on polar stratospheric clouds present at higher altitudes and colder temperatures. Because of the very low predicted background abundances of CIO near the tropopause, such reactions could enhance the local CIO mixing ratios by up to 30-fold at midlatitudes. Substantial perturbations are also predicted for related chemical species (e.g., HCl, HOCl, ClONO2, NO2, HO2) in the midlatitude and tropical tropopause regions due to these heterogeneous reactions. If cirrus clouds occur with sufficient frequency and spatial extent, they could influence not only the chemical composition but also the ozone depletion in the region near the tropopause. Because of variations in observed cloud occurrence frequency and in photochemical and dynamical timescales, the presence of cirrus clouds likely has its largest effect on ozone near the midlatitude tropopause of the northern hemisphere in summer.¿ 1997 American Geophysical Union