The acidity of acetylene CH is stronger than that of alkane CH, and the attractive interaction between an acetylene CH with ƒÎ-electrons, which shows a clear hydrogen bond property, is called activated CH/ƒÎ interaction. In this study, cooperative enhancement of the activated CH/ƒÎ interaction has been probed through the cluster size dependence of the red shift of the acetylene CH stretching vibrational band in neutral phenol-(acetylene)n (∼16 ≤ n ≤ ∼30) and (acetylene)n+ (10 ≤ n ≤ 70). In both the clusters, the characteristic asymmetric (red-shaded) shape of the CH stretch band has been observed. This band shape means that the magnitude of the activated CH/ƒÎ interaction is enhanced by its cooperativity in the interior moiety of the cluster. The red-shifted component of the band extends with increasing cluster size, and the edge of this component seems to reach to the CH stretch band position of crystalline acetylene at the size of n = 20?30, indicating that dozens of molecules need to interact each other to maximize cooperativity in the activated CH/ƒÎ interaction of acetylene. On the other hand, the peak position of the band does not converge to that of crystalline acetylene in the observed size range. The present result suggests that the spectral convergence of acetylene clusters to the bulk may occur in the cluster size range of hundreds or larger. -Close-