A diblock copolymer of polybutadiene and poly(ethylene oxide) has been spread at the interface between cyclohexane and water, and the properties of the capillary waves were determined by light scattering. As the concentration of block copolymer at the interface increases, the capillary wave frequency and damping decrease at a rapid rate initially but tend to asymptotic values at interfacial concentrations of 2 mg m(-2) and greater. Analysis of the correlation functions of the scattered light for the interfacial viscoelastic moduli (interfacial tension, dilational modulus, and dilational viscosity) required the incorporation of the dilational terms to obtain a fit to the data that displays uncorrelated residuals. Interfacial tensions obtained by light scattering were in reasonable agreement with those obtained from interfacial pressure isotherms and exhibited the same dependence on the interfacial concentration of the copolymer. The dilational moduli from the light scattering data were in rather poor agreement with the values from isotherm data, and this is attributed to the low sensitivity of the light scattering data to the dilational modulus because of the reduced coupling at liquid-liquid interfaces. The dilational viscosity however had the same characteristics that have been observed for graft copolymers at the air-water interface, and a Kramers-Kronig type relation has been invoked to explain the observations in a qualitative manner. The frequency dependence at fixed copolymer concentration of the interfacial tension and dilational modulus displays no characteristic behavior, but the dilational viscosity shows an extremely regular dependence on the capillary wave frequency. The behavior can be explained in a qualitative way by. invoking relations used for surfactant solutions where a barrier to surface adsorption is postulated.