Reactions of anionic water clusters of the type X(-)(H2O)(n), (n = 1-50, X = OH and O) with hydrochloric acid have been investigated under single-collision conditions in a Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer. For all the clusters studied, the first step consists in replacement of the anionic X(-) core with a chloride anion. A series of subsequent ligand exchange and fragmentation reactions result in terminal anions of the types Cl-(HCl)(n) and Cl-2(-)(HCl)(n) in different proportions. The detailed reaction sequence is discussed, and formation of free OH radicals in the reactions of O-(H2O)(n) anions is detected. To gain some qualitative insight into the anion geometry and structure, a series of ab initio calculations by a density functional approach was performed for selected clusters.