A quasi-elastic laser scattering (QELS) method was applied to monitoring the capillary wave frequencies at a water/ nitrobenzene interface. A digital spectrum analyzer was adopted to improve the time resolution of the QELS method. As a result, each power spectrum could be obtained in 1s and saved in an additional 1s. The equipment was checked by monitoring capillary wave frequencies at a water/nitrobenzene interface covered with a monolayer of dipalmitoylphosphatidylcholine (DPPC), i.e. changes of interfacial tension caused by the formation of the monolayer at the interface were detected. The equipment was then applied to a chemical oscillation system. It was found that the interface of the chemical oscillation system was covered with a high density membrane during the chemical oscillation.