Laboratory studies of sticking coefficients of H2O, HCl, Cl2and HNO3on ice and heterogeneous reactions of ClONO2with ice or HCl/ice have been performed in a fast flow reactor. A quadrupole mass spectrometer with electron impact ionization was used as a detector. Measured sticking coefficients are: 0.3 (+0.7. −0.1) for H2O, 0.4 (+0.6, −0.2) for HCl,<1.0 × 10−4for Cl2, and 0.3 (+0.7, −0.1) for HNO3at 200 K. The reaction probability of ClONO2on ice was found to be 0.06 (±0.03) while HOCl was observed as a sole product in the gas phase. With HCl present in ice, the reaction probability of ClONO2is greatly enhanced, approaching 0.27 (+0.73, −0.13) while molecular chlorine was found to be the major product in the gas phase. Another reaction product was nitric acid which remained in the solid phase. Since the polar stratospheric clouds contain ice particles or possibly HCl/ice particles on the surface, the present results should be a major factor in producing the observed springtime ozone depletion in the Antarctic s