The dihydrogen bonding (DHB) between the hydride atom of the [Bu4N][GaH4] salt and different XH acids, namely CF3CH2OH, FCH2CH2OH, iPrOH, MeOH, indole and 4-O2NC6H4NH2, has been investigated for the first time by low-temperature IR spectroscopy. The DHB spectroscopic and thermodynamic parameters in solvents of low polarity and high basicity (Ej = 1.37) were obtained. The nature, structures, energies and electron distributions of the model DHB complexes of GaH4− with H2O, MeOH, CF3OH, H2NCF3 or NO2C6H4NH2 were studied by DFT calculations and compared with those of the analogous complexes of BH4− with H2O, MeOH or CF3OH. Two types of DHB structures with monodentate and symmetric chelate coordination were found. The lengthening of the X−H and Ga−H bonds, electron polarization and overlap population of the H···H bond are greater in the first type of structure. A comparative analysis of experimental data demonstrates that the spectroscopic and thermodynamic characteristics, e.g. ν(XH) and ν(GaH) band shifts, ν(XH)bonded integral intensities, formation enthalpies and Ej of all the DHB complexes increase down the group (BH4−·HX < GaH4−·HX). (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)