THE direct reaction of HOC1 with HC1, known to occur in liquid water and on glass surfaces , has now been measured on surfaces similar to polar stratospheric clouds and is shown here to play a critical part in polar ozone loss. Two keys to understanding the chemistry of the Antarctic ozone hole are, one, the recognition that reactions on polar stratospheric clouds transform HC1 into more reactive species denoted by ClO (refs 812) and, two, the discovery of the ClO-dimer (C1 O ) mechanism that rapidly catalyses destruction of O (refs 1315). Observations of high levels of OClO and ClO in the springtime Antarctic stratosphere confirm that most of the available chlorine is in the form of ClO (refs 20, 21). But current photochemical models have difficulty converting HC1 to ClO rapidly enough in early spring to account fully for the observations . Here I show, using a chemical model, that the direct reaction of HOC1 with HC1 provides the missing mechanism. As alternative sources of nitrogen-containing oxidants, such as N O and ClONO , have been converted in the late autumn to inactive HNO by known reactions on the sulphate-layer aerosols , the reaction of HOC1 with HC1 on polar stratospheric clouds becomes the most important pathway for releasing that stratospheric chlorine which goes into polar night as HC1. © 1992 Nature Publishing Group. 1 2 3,4 5-7 1619 5-7,20,21 24-27 x 2 2 3 x 22,23 x 2 5 2 3