Results of laboratory simulations of the growth of nitric acid trihydrate, HNO3·3H2O, on sulfuric acid tetrahydrate, H2SO4·4H2O, are presented. The observations reveal that under typical stratospheric conditions uptake of HNO3 on a H2SO4·4H2O substrate results in a surface coverage of approximately one monolayer or less, and that initial HNO3·3H2O nucleation requires a large supersaturation. We also observe that a H2SO4·4H2O substrate, onto which a HNO3·3H2O film has been deposited and subsequently evaporated, exhibits a remarkable enhancement in its nucleation ability for this nitric acid hydrate. In the stratosphere, PSC particles may experience repeated cycles of evaporation and condensation of HNO3 on preexisting background frozen sulfate aerosols. Hence, growth of HNO3·3H2O on preactivated aerosols provides one important mechanism for polar stratospheric cloud formation and denitrification. Copyright 1996 by the American Geophysical Union.