In this paper the kinetics of the reaction of [IrCl5(H2O)](2-) with CH3CN has been studied. The reaction product was isolated and identified as K-2[IrCl5(CH3CN)]. 2H(2)O. The experimental rate constants show no constant order with respect to the acetonitrile concentration. A preferential solvation of [IrCl5(H2O)](2-) by acetonitrile in water-acetonitrile mixtures is suggested to explain the data. The obtained optical absorption spectra of [IrCl5(CH3CN)](2-) are in good agreement with data already published, but another interpretation is given for the observed d-d transitions. The low energy transition at about 24,000 cm(-1) is predicted to be a singlet-singlet transition ((1)A(2) <-- (1)A(1)), and not a singlet-triplet transition as proposed in other work. (C) 1998 Elsevier Science Ltd. All rights reserved.