Complexation of FeIII ion with thiocyanate in dichloromethane–methanol solvent mixture was studied because of our interest in the analytical uses of FeII-to-FeIII transformations in non-aqueous media. Molar absorptivity and composition of the complex, as well as the solvolysis constant and the stability constant of the complex as a function of the solution composition were determined by spectrophotometry. The composition of the complex throughout the solubility range of the components was that of a monocomplex, [FeNCS]2+. In highly acidic solutions, molar absorptivity of the complex was not influenced by acid concentration and was (13 700 ± 700) dm3 mol-1 cm-1. The solvolysis constant was similar to the value in water, Ks = 7.0 × 10-3 mol dm-3. The stability constant of the complex, on the other hand, was strongly influenced by acid concentration. Extrapolation to the infinite acid concentration yielded the stability constant 40 500 dm3 mol-1, which is adequate for analytical uses of the complex in non-aqueous media
