The dynamic behavior of the phase transfer catalyst, quaternary ammonium bromide R4N+Br- (R: n-C4H9, n-C3H7, C2H5), at a water/nitrobenzene (W/NB) interface was monitored by time-resolved quasi-elastic laser scattering (QELS) measurements. The time courses of the QELS spectrum after injection of the (n-C4H9)(4)N+Br- phase transfer catalyst, which brings about high product yield, showed high interfacial molecular number density in the stationary state of the reaction, whereas its adsorption rate in the initial stage was almost the same as the rates of the other two. The product yield was considered to reflect the interfacial amount of the phase transfer catalyst, and the interface acted as the reaction site controlling the phase transfer catalytic reaction.