The aim of the study was to investigate the kinetics and mechanisms of the replacement of fluoro-ligand from a series of complexes in aqueous acidic solutions. The compounds studied were all octahedral complexes of chromium (III) or cobalt (III). The kinetic runs were studied by a spectrophotometric method and by direct estimation of free fluoride ion concentrations. All reactions were first order in complex ion concentration. The rate constants were all acidity function dependent. In no case was a maximum rate constant observed, even at very high acid concentrations. For some of the complexes rapid reversible changes of the visible and ultraviolet spectra are observed when the solutions are alternately carefully acidified and diluted. These spectral changes may be attributed to a rapid reversible protonation of the fluoro-ligand. The apparent ionisation ratios of the protonated species have been evaluated. The initial product of the hydrolysis reactions is the corresponding aquo (or aquofluoro) complex. For some of the compounds studied the aquofluoro species have been isolated for the first time. At later times some of the aquo-fluoro-species are anated by e.g. nitric or sulphuric acid. The kinetic results have been analysed according to the postulates of Zucker and Hammett and of Bunnett and of Bunnett and Olsen. The chromium complexes all appear to react via a mechanism in which water is involved as a proton transfer agent. In the case of a cobalt complex the water molecule is involved as a nucleophile. The reactions do not exhibit general acid catalysis but fluoro species (e.g. HF 2-, HF or F-) do catalyse the replacement of the fluoro-ligand. This catalysis is attributed to the unusual properties of hydrogen fluoride.