Densities and viscosities have been measured for the binary mixtures of acetonitrile and methanol as a function of mole fraction at temperatures of 298.15, 308.15, and 318.15 K. The excess molar volumes V-E derived from the experimental density data and the deviation of the viscosity of mixtures from the ideal mole fraction rule Delta eta are negative at all the temperatures investigated. The V-E and Delta eta values have been fitted to a Redlich-Kister-type polynomial relation to estimate the coefficients and standard deviations. The results are used to interpret the nature of interactions.