The rates of change in film pressure, Pi, of proteins during the adsorption of human serum albumin, gamma-globulin, lysozyme, myoglobin, concanavalin A, and beta-casein at the air-water interface were monitored using the Wilhelmy plate method. Assuming that unfolding of an adsorbed protein is initiated at the surface of an aqueous solution through the reaction of surface-activated water with the polar internal bond of the protein, the number of water molecules, n, causing a loss in local rigidity of the protein interior was estimated from the kinetic data of protein adsorption. The magnitude of the n values for these proteins with different secondary structures were closely correlated with the kinetic data of the hydrogen-deuterium exchange reaction in protein solution. The present results suggested that the driving force inducing a loss in the internal structure of proteins would be the access of water into the interior of the protein.