- The adsorption kinetics at the air/water and phospholipid monolayer/water interfaces was determined by measuring the change of surface radioactivity of tritium-labeled prothrombin with time. The adsorption kinetics on the mercury electrode, at different polarizations, was determined by measuring the change in capacitance with time, assuming proportionality between capacitance and surface concentration. An inflection point, at which the adsorption rate at the mercury/water interface starts increasing again, is observed at a particular constant surface concentration, irrespective of adsorption time from the different bulk concentrations. Representation of cooperative adsorption, indicated by this behavior, was tried by the Frumkin adsorption isotherm and by a model assuming surface gelation proceeding over a narrow range of surface concentrations. Solution of the diffusion differential equation with the Frumkin adsorption isotherm as boundary condition could not be fitted to the experimental data, while the surface gelation model could be fitted very easily. Best fits were obtained for gelation starting at surface coverage θ = 0.42 and being completed around θ = 0.5.