Structural parameters and IR spectra of p-hydroxyphenyldiazonium cation and its conjugated base and of their monohydrates in the free state and in aqueous solution were calculated by the B3LYP/6-311G**method. The effects of aqueous medium were taken into account using the polarizable continuum model (PCM). The conjugated base is not a quasiaromatic p-diazoniophenolate zwitterion but has an essentially quinoid structure of 4-diazocyclohexa-2,5-dien-1-one. Its dipole moment in water (8 D) is much lower than that of p- diazoniobenzenesulfonate zwitterion (24 D).