The structure of acetonitrile-water mixtures has been investigated by X-ray diffraction with an imaging plate detector and IR spectroscopy over a wide range of acetonitrile mole fractions (0.0 less than or equal to X-AN less than or equal to 1.0) Reichardt E-T(N) and Sone-Fukuda D-II,D-I values were also measured for the mixtures. It has been found from the X-ray data that in pure acetonitrile an acetonitrile molecule interacts with two nearest neighbors by antiparallel dipole-dipole interaction together with a small shift of the two molecular centers and that two acetonitrile molecules in the second-neighbor shell interact with a central molecule through parallel dipole-dipole interaction. Thus, acetonitrile molecules are alternately aligned to form a zigzag cluster. On addition of water into pure acetonitrile. water molecules interact with acetonitrile molecules through a dipole-dipole interaction in an antiparallel orientation. The IR spectra of O-D and C=N stretching vibrations, observed for mixtures of acetonitrile AN and water containing 20% D2O, suggested that hydrogen bonds are also formed between acetonitrile and water molecules in the mixtures at X-AN less than or equal to 0.8. The average numbers of the first- and second-neighbor acetonitrile molecules gradually increase with increasing water content with an almost constant first-neighbor distance and slightly decreased second-neighbor ones, Thus, acetonitrile molecules are assembled to form three-dimensionally expanded clusters, the acetonitrile clusters are surrounded by water molecules through both hydrogen bonding and dipole-dipole interaction The X-ray radial distribution functions and IR spectra suggest that the hydrogen bond network of water is enhanced in the mixtures at X-AN < 0.6. The concentration dependence of ETN and DII,I values determined reflects well the above-mentioned behavior of water molecules in the mixtures. These findings suggest that both water and acetonitrile clusters coexist in the mixtures in the range of 0.2 less than or equal to X-AN < 0.6, i.e., "microheterogeneity" occurs in the acetonitrile-water mixtures.