Thermally excited capillary waves of the poly(ethylene oxide)-block-poly(dimethylsiloxane) copolymer surfactant solutions have been studied by surface light scattering (SLS). The surface tension measured by the Wilhelmy plate method agrees well with that calculated from the surface light scattering data using the Kelvin equation at low and high concentrations, while the deviation is observed in the concentration range between 4 x 10(-7) to 2 x 10(-6) g/mL. This deviation is considered as due to surface adsorption and relaxation in the monolayer. The surface elastic modulus (epsilon’) deduced from the SLS spectrum exhibits a maximum at a concentration around 7.8 x 10(-7) g/mL. The epsilon’ predicted by the diffusion exchange model of Lucassen shows a serious discrepancy with our data. In addition, the epsilon " value is found to be negative in some regions and it displays a minimum for concentrations below 3 x 10(-6) g/mL. The negative values of epsilon " have also been previously reported, and it may be due to the inadequacy of the dispersion equation currently used to interpret the SLS result.