It has been found possible to understand the observed relative abundances of the several different tautomers and conformers observed in the millimeter-wave jet spectroscopy of histamine by using ab initio energy calculations at the MP216-311++G(d,p) level and including an estimate of thermal free energies. Conformational relaxation, previously found to be import-ant for several molecules of comparable complexity, has been found to have a significant role in determining the relative abundances of histamine species detectable in the cooled jet, the present example being indicative that this phenomenon is expected to influence generally the interpretation of multiconformational jet spectra. The need to include free-energy corrections is also noteworthy. The thermal equilibration of histamine tautomers in the gas phase within the millimeter-wave spectrometer prior to jet expansion has been confirmed experimentally by comparing abundances measured with high surface area and low surface area inlet systems. Thus, it is valid to use relative abundances to indicate relative free energies. The presented ab initio calculations yield spectroscopic constants (e.g., planar moments) that tighten the identification of the four species detected in jet spectroscopy. In particular, the identity of the least abundant species (3 G-Ic) is confirmed. Nitrogen quadrupole coupling constants computed at the MP216-311++G(d,p) level are in noticeably better agreement with experiment than previously reported calculations, it being noteworthy that they require no scaling for the N-14 nuclear quadrupole moment.