High resolution Fourier-transform ion cyclotron resonance (FT-ICR) mass spectroscopy is used to investigate reactions of large ionic water clusters H+(H2O)(n) and X(-)(H2O)(n) (n=1-100, X=0 or OH). Reactions of the clusters with chlorine nitrate, important "reservoir compound" involved in the stratospheric ozone chemistry, are investigated to evaluate the importance of heterogeneously catalyzed reactions for ozone depletion. It is found that reactions of both cationic and anionic clusters result in effective hydrolysis of chlorine nitrate and return of the more active hypochlorous acid, HOCl into the gas phase. The chemistry of clusters is discussed, and its validity and relevance as a model for "real life" processes in the so-called polar stratospheric clouds (PSC’s) is assessed.