The present work aims to resolve theoretically the puzzling effect of downshifting of the ν OH stretching mode of phenol, complexed with acetonitrile, as the latter concentration increases, via performing a thorough search of the potential energy surface of the interaction of phenol with acetonitrile at the modest MP2/6-31+G(d,p), MP2/6-31G(d), and B3LYP/6-31+G(d,p) computational levels. We postulate and firmly approve that, besides the conventional σ hydrogen-bonded structure of phenol with one acetonitrile molecule, there exists the novel, although less favorable, structure with π-type hydrogen bonding. Such bonding becomes crucial as the second acetonitrile molecule is added to the 1:1 complex of phenol and acetonitrile, and furthermore, its substantial stabilization in polar solvents provides an explanation of the mentioned controversial issue that has stood unclear during the last four decades.