Dimeric gaseous species Sn2BrxI4-x, = 0-4, were detected by Knudsen effusion mass spectrometry. Fragment ionic species Sn2BryI3-y+, y = 0-3, yielded by electron bombardment were identified in mass spectra of SnBr2-SnI2 equilibrium vapors. The structure and bonding of identified ions and neutral precursors were studied by quantum-chemical methods. Enthalpies and entropies of the anion-exchange reactions involving gaseous dimeric species were estimated by the use of mass spectrometric and theoretical data. Implications of the thermodynamic and structural data obtained by theoretical methods on the observed fragmentation paths of the neutral Sn2BrxI4-x species are discussed. The nature of interactions leading to dimer formation is studied on the basis of the atomic charge distribution and the variational-perturbational interaction energy decomposition scheme. The studies unravelled the additivity of properties such as bond length, vibrational frequencies, and atomic charge distribution corresponding to Sn-Br and Sn-I bonds participating in studied molecules and ions.