The solubility of i-butyl bromide in nitrobenzene have been measured at 19, 25 and $40^{circ}C$ in the presence and absence of gallium bromide. In the presence of gallium bromide, 1 : 1 complex, $i-C_4H_9Br{cdot}GaBr_3$ is formed in the solution. The instability constant K of the complex formation was evaluated from the following equilibrium equation. $i-C_4H_9Br{cdot}GaBr_3{ ightleftharpoons}C_4H_9Br + 1/2Ga_2Br_6.$ 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.