We report the frequency-dependent absorption coefficient and index of refraction in the far-infrared region of the spectrum for mixtures of acetonitrile and water. The mixtures do not behave ideally, and deviate from ideality most noticeably for mixtures that are between 25% and 65% acetonitrile by volume. Two implementations of the Debye model for describing the dielectric relaxation behavior of mixtures are compared, and we show that these mixtures are better treated as uniform solutions rather than as two-component systems. We find an enhanced structure in the mixtures, relative to ideal mixtures, but we do not find direct evidence for microheterogeneity. The Debye time constant for the primary relaxation process for the mixtures is up to 25% longer than that for an ideal mixture.