Molecular structures of series of InBr3Pyx complexes (x = 1-3) in the solid state have been determined by single crystal structure analysis. For x = 2, an unexpected dimeric In2Br6Py4 structure, which features a nearly planar In2Br6 unit, has been established. This structure completes the series of known valence-isoelectronic dimeric molecules of group 17 (I2Cl6) and group 15 elements (As2Cl6 center dot 2PMe(3)). Theoretical studies at the B3LYP/def2-TZVP level of theory reveal that all gaseous M2X6Py4 dimers (M = Al, Ga, In, Tl; X = Cl, Br) are energetically unstable with respect to dissociation into MX3Py2 monomers. This finding is in stark contrast to the valence-isoelectronic group 17 and 15 analogs, which are predicted to be energetically stable with respect to dissociation. Thus, additional interactions in the solid state play a crucial role in stabilization of the experimentally observed dimeric In2Br6Py4. Thermal stability and volatility of InBr(3)Pyx complexes have been studied by tensimetry and mass spectrometry methods. Mass spectrometry data indicate that, in contrast to the lighter group 13 element halides, species with two In atoms, such as In2Br6Py2, are present in the gas phase. Thermodynamic characteristics for the heterogeneous dissociation processes of InBr(3)Pyx (x = 2, 3) complexes with Py evolution have been determined.