The addition of water to tetrabutylammonium bromide (TBABr)/benzene/sodium bromide ternary component systems induced the separation of TBABr-rich liquid phase (L phase) prior to the appearance of an aqueous phase (W phase). The L phase was assumed to be a bicontinuous phase similar to a microemulsion based on the phase composition. TBABr catalyzed the "solid-liquid" phase-transfer reaction of decyl methanesulfonate with sodium bromide either when the L phase was fully hydrated or after a distinct W phase was formed. The phase-transfer catalytic activity was correlated to the phase behavior of TBABr/benzene/water/NaBr four-component system. The difference and similarity between TBABr and tetrabutylphosphonium bromide (TBPBr) were discussed based on the microstructure of the phases formed by TBABr and TBPBr under the phase-transfer catalytic reaction conditions. (c) 2005 Elsevier B.V. All rights reserved.