The solubilities of i-propyl bromide in nitrobenzene have been measured at 10$^{circ}$, 19$^{circ}$ and 25$^{circ}C$ in the presence and absence of gallium bromide. In the presence of gallium bromide, 1 : 1 complex, i-C$_3$H$_7Br{cdot}GaBr_3$ is formed in the solution. The instability constant K of the complex formation was evaluated from the following equilibrium equation: i-C$_3$H$_7Br{cdot}GaBr_3$ ${ ightleftharpoons}$ i-C$_3$H$_7$Br + $frac{1}{2}$$Ga_2Br_6$. The change of enthalpy, free energy and entropy for the dissociation of the complex were also calculated. From these result, it seems that the stabilities of the complex formation, gallium bromide with alkyl bromide, are directly related with those of the carbonium ions of alkyl bromide.