Solvation of Co(II), Ni(II), Cu(II) and Zn(II) ions in acetonitrile-water mixtures has been studied by infrared and visible spectroscopy. CN stretching vibration of deuterated acetonitrile has been used to determine the number of acetonitrile molecules in the first solvation sphere of the Zn(II) ion as a function of solvent composition. The number of independent solvation complexes present in solutions of Co(II), Ni(II) and Cu(II) perchlorates over the whole range of solvent composition has been obtained from visible absorption spectra. The infrared and visible spectroscopy studies indicate the preferential hydration of the ions examined. Molecular aspects of inner- and outer-sphere solvation have been discussed.