The relationship between the concentration of a phase-transfer catalyst, tetrabutylammonium bromide (TBAB), and the rate of its cyclic reaction was investigated using the quasi-elastic laser scattering (QELS) method. TBAB forms an ion pair (TBA(+)C(6)H(5)O(-)) with sodium phenoxide (C6H5ONa) at the interface. This ion pair moves from the interface to the organic phase where it reacts with diphenylphosphoryl chloride and produces triphenyl phosphate. During this reaction, tetrabutylammonium chloride, which is generated simultaneously with the product, returns to the water phase. An increase in the concentration of the phase-transfer catalyst (TBAB) dose not promote the cyclic reaction. The QELS study indicates that this is because some of the TBAB catalyst adsorbs on the interface and disturbs mass transfer of the other chemical species (TBA(+)C(6)H(5)O(-)). Interfacial coverage of the catalyst should be taken into account to find the optimum concentration of the catalyst. This paper shows for the first time that the interfacial catalyst promotes the reaction and at the same time disturbs mass transfer of the other chemical species.