In this work we have studied the impact of the competitive adsorption of a whey protein concentrate (WPC) and three well characterized hydroxypropyl-methyl-cellulose (HPMCs) commercially known as, E4 M, E50LV and F4 M, on the surface dilatational properties (surface dilatational modulus, E, surface dilatational elasticity, E(d), and loss angle tangent, tan delta) of mixed films adsorbed at the air-water interface. The increase in E(d) values with adsorption time could be associated with biopolymer adsorption at the interface. The surface dilatational properties depend on the WPC and HPMC concentrations in the aqueous phase and on the WPC/HPMC ratio. Although the values of E(d) were mainly determined by HPMC at short adsorption times, for mixed systems with the lowest protein concentration (1 x 10(-4) wt%) the E(d) values were close to those of HPMCs, even at long term adsorption. The values of tan delta indicate the formation of adsorbed mixed films with high viscoelasticity, with a gel structure, which in turn should be attributed to the association of biopolymer molecules occurring at the interface. Only one biopolymer is the dominant one in the solid character of these mixed systems. HPMC at high concentrations slightly reduced the long-term solid character of the films confirming the existence of competition for the air-water interface as expected with two surface-active biopolymers with high molecular weight. (C) 2009 Elsevier Ltd. All rights reserved.