Attempt has been made to explain why different aliphatic quaternary ammonium tribromides (QATBs) show different levels of efficiency in bromination reactions. For this study, tetrabutyl ammonium tribromide (TBATB, [N(C4H9)(4)]Br-3), and cetyl trimethyl ammonium tribromide (CTMATB, [N(CH3)(3)C16H33]Br-3) were used as representative examples. UV-Vis Spectroscopic method was employed to measure the thermodynamic parameters of the reactions. Influence of quaternary ammonium cation on tribromide (Br-3(-)) performance was assessed with the aid of Density Functional Theory (B3LYP/6-311 + + G(d,p)) by determining the dissociation energy of the QATBs, energies of pairs of their frontier orbitals, which are HOMO and LUMO and calculation of dipole moments.