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Abstract: The effect of introducing water molecules into a Cxb7;·O complex on the interaction between the methyl hydrogen atom of dimethylsulfoxide (DMSO) and the oxygen atom of H2O has been investigated using the MP2/6-311++G(d,p) method to explore how the binding strength of the weak hydrogen-bond Cxb7;·O interaction changes during hydration. The calculations in gas phase, including the interaction energies, binding distances, stretching frequencies, isotropic shielding constants, natural population analysis (NPA) charges, and stabilization energies, on the DMSO–H2O complex showed that the Cxb7;·O interaction is enhanced by introducing water molecules into the system, and the effect of first water molecule is more prominent than that of the second. The results obtained in liquid media support these conclusions. The cooperativity between Oxb7;·O and Cxb7;·O hydrogen-bonds was also evaluated. The results demonstrate that both such cooperativity and solvation play important roles in enhancing the Cxb7;·O hydrogen bond in DMSO–H2O complex. The results are helpful to better understand the role of water molecules in biological processes involving Cxb7;·O interactions. [Copyright &y& Elsevier]