The molecular geometry and vibrational frequencies of borane ammoniate, BH3-NH3, are studied by ab initio calculations in the solid state. The B-N bond distance shortens by similar to0.08 Angstrom, and the corresponding stretching frequency increases by 200 cm(-1), compared to the same characteristics of the molecule in the gas phase. A reassignment of the experimental argon-matrix vibrational spectrum in an earlier work [Smith, J.; Seshadri, K. S.; White, D. J. Mol. Spectr. 1973, 45, 327-337], on the basis of these calculations, unifies all the experimental vibrational data available for this molecule. The shortening of the B-N bond is analyzed in terms of the atoms-in-molecules theory.