Specific interaction of N-octadecanoyl-N-phenylhydroxylamine (ODPHA) and GA(3+) was evaluated by means of the L-B technique. The monolayer of ODPHA on a subphase containing Ga3+ and/or Al3+ was stable, Limiting areas of monolayer of ODPHA on the subphases are increased with pH of the subphase and reach constant values at certain pHs, 2.5 (on the Ga3+ solution) and 4.5 (on the Al3+ solution), respectively, The surface pressure-area isotherm for ODPHA on the Ga3+/Al3+ solution containing a 388-fold amount of Al3+ at pH 2.4 was similar to that for ODPHA on the Ga3+ solution at the same pH, A microporous polymeric membrane impregnating ODPHA showed a selective separation of Ga3+ from a solution containing a 388-fold amount of Al3+ at pH 2.4, The separation factor toward Ga3+ reached over 300.