Mixtures of water and acetonitrile with trivalent cations-Al3+. Fe3+, and Cr3+-in the full concentration range have been studied by infrared spectroscopy. CN stretching vibrations of the CD3CN molecules have been used as probes of the structural environments. The CN band in each case is a superposition of four subbands, which may be attributed to the CD3CN bonded in the first, second, and third coordination shells of the cation and to the free acetonitrile. The changes of the integral intensities of the subbands with the H2O:Me3+ molar ratio show that water tends to dislodge acetonitrile from the coordination spheres of the cation. The composition of the first coordination sphere is determined only by the H2O:Me3+ molar ratio, whereas the composition of the second and third spheres depends on the concentration of both solvents.