Activation reactions of benign chlorine species (HCl, CIONO2) on aerosols in the winter polar stratosphere set the stage for the spring-time catalytic destruction of ozone leading to the Antarctic ozone hole. Field observations have demonstrated the existence of both solid and liquid particles consisting of H2SO4, HNO3, and H2O. The exact freezing conditions and final composition of the solid aerosols remain the subject of investigations. We present laboratory observations of isolated individual sulfuric acid/water particles under stratospheric temperatures and water vapor pressures. Our experiments demonstrate that this binary system would not freeze unless temperatures were below the water-ice frost point. Upon freezing, we observe H2SO4-8H2O, not the generally invoked H2SO4-4H2O. We suggest that the water-rich octahydrate phase is likely to be one of the high relative humidity forms which is efficient in chlorine activation.¿ 1997 American Geophysical Union