We present calculations for the acetonitrile - water clusters to examine the nature of interactions in the mixed clusters. We calculate conformers of various composition, either of σ -type (-OH and -CN binding linearly) or π -type (-OH and -CN interacting perpendicularly) structures for the acetonitrile - water clusters. We predict that the IR frequency of the proton-accepting C≡N stretching mode red-shifts in the σ -type clusters and blueshifts in π -type conformers, whereas the proton-donating ?OH stretching frequency red-shifts in all cases. We find that this intriguing pattern also applies to the acetonitrile - water clusters of various molar ratio.