Non-equilibrium interfacial layers formed by competitive adsorption of β-lactoglobulin and the non-ionic triblock co-polymer PEO99-PPO65-PEO99 (F127) to the air-water interface were investigated in order to explain the influence of polymeric surfactants on protein film surface rheology and foam stability. Surface dilatational and shear rheological methods, surface tension measurements, dynamic thin-film measurements, diffusion measurements (from fluorescence recovery after photo bleaching) and determinations of foam stability were used as methods. The high surface visco-elasticity, both the shear and dilatational, of the protein films was significantly reduced by co-adsorption of polymeric surfactant. The drainage rate of single thin films, in presence of β-lactoglobulin, increased with the amount of added F127, but equilibrium F127 films were found to be thicker than β-lactoglobulin films, even at low concentration of the polymeric surfactant. It is concluded that the effect of the non-ionic triblock copolymer on the interfacial rheology of £]-lactoglobulin layers is similar to that of low molecular weight surfactants. They differ however in that F127 increase the thickness of thin liquid films. In addition, the significant destabilising effect of low molecular weight surfactants on protein foams is not found in the investigated system. This is explained as due to that long-range steric forces start to stabilise the foam films at low concentrations of F127.