The infrared and Raman spectra of histamine dihydrochloride and its N-deuterated derivative were recorded in the solid state. Raman spectra of samples dissolved in both normal and heavy water were also obtained. A general assignment of the fundamental vibrations of this cationic species of the histamine molecule was proposed on the basis of the isotopic shifts and correlations with similar molecules. Wavenumber assignments to the imidazole stretching vibrations indicate a small increase in the ring electronic density with respect to histamine monocation, although the isotopic shifts involved lead to some differences between the two sets of normal modes. The deviations observed on comparing the Raman spectra of solid and dissolved histamine dication are not significant, agreeing with the fact that the two imidazole nitrogen sites are occupied. Copyright © 2000 John Wiley & Sons, Ltd.