We have used mechanically generated capillary wave and ellipsometric techniques to investigate interfacial viscoelastic properties of adsorbed monolayers of polystyrene-6-poly(methacrylic acid) diblock copolymer at an air-water interface, as a function of both the overall molecular weight, M(W), and the nominal interfacial number density of the copolymer. This experiment is a follow-up of our earlier experiment, in which we studied adsorbed monolayers of the same diblock copolymer at the toluene-water interface (Macromolecules 1993, 26, 6595). Now we have changed the environment of the polystyrene block from toluene to air and have studied the effect of such a change. The most prominent effect of this change is that it is more difficult to attain equilibrium at the air-water interface. Unlike the toluene-water case, no clear saturation of surface pressure is observed at the air-water interface. The maximum surface pressure values measured at the air-water interface are smaller than the saturation surface pressure values in the toluene-water case for all the three molecular weights we have investigated. Ellipsometric study shows that only a very small fraction of the copolymer molecules added to the system is adsorbed at the air-water interface. However, substantial changes in the longitudinal elasticity and longitudinal viscosity are observed.