The real and imaginary refractive index spectra of mixtures of water and acetonitrile over the full composition range at 25 degrees C were determined between 8000 and 700 cm(-1) by calibrated multiple attenuated total reflection spectroscopy. Under the assumption of the Lorentz local Geld, the corresponding molar polarizability spectra, <(alpha)over cap (m)(<(nu)over tilde>> = alpha’(m)(<(nu)over tilde>) + i alpha "(m)(<(nu)over tilde>), were calculated and used to investigate the structure of the mixtures. The concentrations of water-bonded, acetonitrile-bonded, and non-hydrogen-bonded O-H groups, and of water-bonded and non-hydrogen-bonded acetonitrile molecules, were obtained from the integrated intensities C-OH and C-CN, the areas under the O-H and C=N stretching bands in the <(nu)over tilde alpha "(m)> spectra. The results indicate that no enhancement of the water structure (O-H-O bonding) results from the addition of acetonitrile. In contrast, a monotonic decrease in the fraction of O-H groups that are bonded to oxygen is observed with increasing CH3CN content. At low acetonitrile concentration, x(CH3CN) less than or equal to 0.05, where x is the mole fraction, the total fraction of OH groups that are hydrogen bonded increases slightly with increasing CH3CN content because the formation of OH-N bonds slightly exceeds the destruction of O-H-O bonds. The present results are consistent with the existence of microheterogeneity at compositions near 30-50 mol % of acetonitrile. However the fraction of OH groups that are hydrogen bonded to water is 0.50 at 50 mol % CH3CN and decreases to 0.35 at 70 mol % CH3CN. Both of these fractions are too small to support water clusters more complex than linear chains or hexagons.