The dilational properties of monolayers are analyzed using the classical linear approximation. In most cases, the observed interfacial behavior can be approached by a model corresponding to a two-dimensional viscoelastic solid. The monolayer is characterized by two dilational elasticity terms (E-e, equilibrium elasticity, and E-ne, nonequilibrium elasticity) and by one relaxation time (tau). These three physical constants are obtained from the responses of a ramp type perturbation, or from the responses (as a function of the frequencies) after sinusoidal area variations. Using axisymmetric drop shape analysis experiments. a dipalmitoyl phosphatidylcholine (DPPC) layer at the dichloromethane/water interface is characterized. Measurements of the surface pressure variations as the response to linear or sinusoidal variations of surface area are performed. Identical rheological physical constants (equilibrium elasticity, nonequilibrium elasticity, and relaxation time) are obtained using both methods. Dilational behavior of DPPC monolayer can be attributed to the molecular diffusion between the DPPC layer and the adjacent phases.