Adsorption of pulmonary surfactant to an air water interface lowers surface tension (gamma) at rates that initially decrease progressively, but which then accelerate close to the equilibrium gamma. The studies here tested a series of hypotheses concerning mechanisms that might cause the late accelerated drop in gamma. Experiments used captive bubbles and a Wilhelmy plate to measure gamma during adsorption of vesicles containing constituents from extracted calf surfactant. The faster fall in gamma reflects faster adsorption rather than any feature of the equation of state that relates gamma to surface concentration (Gamma). Adsorption accelerates when gamma reaches a critical value rather than after an interval required to reach that gamma. The hydrophobic surfactant proteins (SPs) represent key constituents, both for reaching the gamma at which the acceleration occurs and for producing the acceleration itself. The gamma at which rates of adsorption increase, however, is unaffected by the Gamma of protein in the films. In the absence of the proteins, a phosphatidylethanolamine, which, like the SPs, induces fusion of the vesicles with the interfacial film, also causes adsorption to accelerate. Our results suggest that the late acceleration is characteristic of adsorption by fusion of vesicles with the nascent film, which proceeds more favorably when the Gamma of the lipids exceeds a critical value.