The nonmonotonic kinetic dependencies of the dynamic elasticity of adsorbed and spread β-casein layers at the liquid-gas interface have been determined by the oscillating barrier method. While two local maxima in the surface elasticity versus concentration dependence are well documented in literature, these features have not been reported for kinetic curve. The surface elasticity in the time range of the second maximum depended on the β-casein bulk concentration and deviated from the elasticity of spread β-casein layers at the same surface pressures. In parallel to the surface viscoelasticity of PEO-PPO-PEO block copolymers, the experimental findings for β-casein can be explained by a separation of relatively hydrophobic groups of the polypeptide chain during the slow process of protein adsorption. © 2007 American Chemical Society.