The surface of a soluble monolayer (Brij-58 solutions) in equilibrium is compressed with a constant speed, and the surface tension with time is measured, To analyze the experimental data the convective diffusion equation is integrated and thereafter approximated. The agreement between theoretical predictions and experimental data was good, using Gibbs elasticity and diffusion relaxation time as adjustable parameters, From these data the elasticity of the monolayer as a function of the effective time is obtained, For a micellar system, the bulk region adjoining the surface becomes oversaturated, after threshold micellization sets in, The Gibbs elasticity and the diffusion relaxation time resulting from experimental data sue compared with those obtained from the adsorption isotherm, The agreement is poor, This is discussed by considering that the Gibbs elasticity and the diffusion relaxation time require second differentials of the equilibrium surface tension concentration curve.