Quasi-elastic laser light scattering from thermally excited capillary waves on the polarised water I 1,2-dichloroethane interface was measured by the optical heterodyne mixing technique. The interfacial potential difference was controlled by equilibrium partition of hexadecyltrimethylammonium. chloride (HTMACl). The observed frequencies and the damping constant of the capillary waves were found to follow the theoretically predicted dependence on the capillary wave number. The interfacial viscosity apparently plays a negligible role in both the absence and the presence of HTMACl The interfacial tension evaluated from the capillary wave frequency at various HTMACl concentrations agrees well with that obtained by the video-image pendant-drop method. Interfacial tension data were used to infer the relative surface excess of HTMACl. Analysis of adsorption data points to a strong specific adsorption of HTMA(+) ion, and to weak repulsive interactions between the adsorbed HTMA(+) ions.