The structures of the molecules methylamine-borane, MeH2N center dot BH3, and dimethylamine-borane, Me2HN center dot BH3, have been investigated by gas-phase electron diffraction (GED) and quantum chemical calculations. The crystal structures have also been determined for methylamine-, dimethylamine-, and trimethylamine-borane, MenH3-nN center dot BH3 (n = 1-3); these are noteworthy for what they reveal about the intermolecular interactions and, particularly, the N-H center dot center dot center dot H-B dihydrogen bonding in the cases where n = 1 or 2. Hence, structures are now known for all the members of the ammonia- and amine-borane series MenH3-nN center dot BH3 (n = 0-3) in both the gas and solid phases. The structural variations and energetics of formation of the gaseous adducts are discussed in relation to the basicity of the MenH3-nN fragment. The relative importance of secondary interactions in the solid adducts with n = 0-3 has been assessed by the semi-classical density sums (SCDS-PIXEL) approach.