Dynamic surface dilational properties of alcoholic solutions of proteins (Bovine Serum Albumine (BSA) and beta-casein) were studied by the damped longitudinal waves (0.1-1 Hz) method as a function of the ethanol content. At low ethanol contents (0-5%, v/v) the BSA solutions and the beta-casein at 12%, v/v, exhibited viscoelastic behavior. At high ethanol content, the BSA solution surface displayed an elastic behavior. Absolute surface theological coefficients (high-frequency elasticity E(T0), low-frequency elasticity E(0), the constant rate of relaxation k(r), and the intrinsic surface viscosity eta(s)) of the surface at its quasiequilibrium were obtained from the frequency spectra of the waves’ characteristics. We have introduced two models : one based on the adsorption-desorption of protein aggregates and another based on a conformational change surface reaction, which could fit our results. The decrease of E(T0), k(r), and eta(s) when ethanol content increases has been related to the decrease of surface coverage by protein.