The interfacial behavior of a phase transfer catalyst, tetrabutylammonium bromide (TBAB), was investigated by the quasi-elastic laser scattering (QELS) method. TBAB forms an ion pair (TBA(+)C(6)H(5)O(-)) with sodium phenoxide (C6H5ONa) in a water phase. This ion pair moves from the water phase to the organic phase where it reacts with diphenylphosphoryl chloride (DPPC). This one process in the phase transfer catalytic reaction at a water/nitrobenzene (W/NB) interface was investigated by the QELS method. When the TBAB concentration was above 50 mM, the ratio of TBAB concentration to C6H5ONa in equilibrium at the W/NB interface was unity. On the other hand, when the TBAB concentration was below 50 mM, the ratio was below unity. These results suggested that the reaction place between;TBAB and C6H5ONa changed with TBAB concentration; the reaction place was in the water phase above 50 mM, whereas it was at the interface below 50 mM.