The Raman spectra of acidified aqueous iron(III) chloride solutions have been measured between 25 and 300 °C. When Fe3+concentrations are in the range 0.75 to 1.0 mol kg−1and Cl−/Fe3+ratios,R, in the range 3 to 9.5, the dominant species at 25 °C istrans-[Fe(H2O)4Cl2]+; at 300 °C the sole iron-containing species is tetrahedral. Conversion of [Fe(H2O)4Cl2]+intoappears not to involve intermediate iron species. In the presence of excess chloride the reactionis presumed to occur; ΔHfor this reaction has been estimated as +65 ± 8 kJ mol−1. In addition to increasing temperature, factors which favourover other iron species include increasing acidity, increasingR, and decreasing dielectric constant.Keywords: high temperature aqueous solutions, iron(III) chloride, Raman spectroscopy.