The dilational rheological properties of interfacial films of poly(ethylene glycol) (PEG2000) and hen egg-white lysozyme (HEWL) were studied respectively at the dichloromethane (DCM)-water and air-water interfaces by means of the pendant drop method. In both cases, the observed interfacial behaviors were approached by a model corresponding to a two-dimensional viscoelastic solid. The interfacial layers were characterized by three physical constants: E-e, the equilibrium elasticity, E-ne, the nonequilibrium elasticity, and tau, the relaxation time. Because the interfacial dilational properties of the films were studied by using a ramp type perturbation approach or a sinusoidal variations approach, identical rheological physical constants values were obtained for PEG2000 and HEWL. From these studies, the interactions within the interfacial layer and those between the interfacial film and adjacent phases can be indirectly accessed and estimated.