Structural and vibrational properties of histamine monocation in aqueous solution have been studied by means Fourier transform vibrational spectroscopies and a continuum model with the B3PW91 functional. Solvent effects were incorporated by means of an ellipsoidal cavity model with a multipolar expansion (up to six order) of the solute＇s electrostatic potential. Calculations were always performed at the 6-31+G* ab initio level. Solutions in water and deuterium oxide were investigated. Discussion was focused on the trans N-tau-H conformer of histamine monocation, which is the predominant structure in water solution. The optimized geometry was compared to that reported for this species in the solid state by diffraction techniques, where the only conformer present is the trans N-tau-H. A general assignment was proposed for the infrared and Raman spectra of histamine monocation in solution, based on the isotopic shifts and a previous vibrational study in solid state. Force field and normal coordinate calculations were computed to support these assignments. The ab initio force constants were transformed into a set of locally symmetrized internal coordinates and subsequently scaled to the experimental frequencies by using one specific and two generic scaling factors. The comparison in terms of vibrational frequencies and normal coordinate descriptions supported most of the proposed assignments. The theoretical infrared spectra for the two isotopomers on the basis of the ab initio intensities also showed a good correlation with the experimental spectra. These results evidence that solute-solvent interactions must be explicitily taken into account in order to understand the vibrational behavior of polar species in solution; in addition, the use of multipolar expansions for the electrostatic potential together with a cavity adapted to the molecular shape improves significantly the performance of the solvation model.