The hydrogen-bonded structure of the (Benzoic acid)₂-(Water)₁ (BA₂-W₁) cluster was investigated by infrared spectroscopy in the OH stretch region. The stable BA dimer has the cyclic structure with the two hydrogen bonds between the carboxyl groups. The observed infrared spectrum of BA₂-W₁ shows that the water molecule is inserted in between the two carboxylic groups of the cyclic dimer. This means that a single water molecule begins to dissociate the cyclic dimer. This observation can be regarded as the initial step of dissociation of the stable cyclic BA dimer in aqueous solution.

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