A polybutacliene-poly(ethylene oxide) diblock copolymer has been spread at the air-water interface, and the surface pressure isotherm has been determined. Over the same concentration range, a spread film of polybutadiene exhibits no surface pressure and thus for the copolymer the surface moduli (surface tension and dilational modulus) are due to the solvated poly(ethylene oxide) block. Surface quasi-elastic light scattering has been used to obtain surface viscoelastic moduli over a wide range of surface concentration at a surface wavenumber of 395 cm(-1). For copolymer surface concentrations below 0.8 mg m(-2), surface tension and dilational modulus obtained in this manner were identical with those derived from the surface pressure isotherm. At higher surface concentrations, the light scattering values exceeded those from the isotherm and consequently relaxation processes take place in the spread film. The capillary wave damping exhibits two resonance points at surface concentrations of ca. 0.7 and 0.9 mg m(-2). The surface wavenumber (q) dependences of the capillary wave frequency, damping, and surface viscoelastic moduli were determined for a copolymer surface concentration of 0.73 mg m(-2) where there is a resonance between the capillary and dilational modes. The surface moduli follow a Maxwell-like behavior for frequencies greater than 2 x 10(5) s(-1), and there is a change in the q dependence of the damping that suggests that dilational behavior dominates at low q and capillary mode behavior dominates at high q. This is suggestive of mode mixing being present.