The excess partial molar enthalpy of 1-propanol (1P), H-1P(E), was measured at 28 ° C in the ternary mixture of 1P-1-butyl-3-methylimidazolium chloride ([bmim]Cl)-H2O in the H2O-rich composition range. From these data we evaluated what we call the 1P-1P enthalpic interaction function, H-1P-1P(E). Its changes induced by addition of (bmim]Cl of the pattern of H-1P-1P(E) were used as a probe to elucidate the effect of [bmim]Cl on the molecular organization of H2O. It was found that the effect of Cl- was not conspicuous within this methodology, and the observed dependence is predominantly due to the hydration of [bmim](+). The changes in the χ(1P)-dependence of H-1P-1P(E) were compared with those brought about by temperature increase, or by the addition of fructose or glycerol. It was found that the effect of [bmim](+) is similar to that of fructose or increased temperature. We speculate that in the H2O-rich composition region a number of H2O molecules are attracted to the delocalized positive charge of the imidazolium ring and the bulk of H2O is influenced in such a manner that the global hydrogen bond probability is reduced.