Interfacial tension changes during protein adsorption at both the solid-liquid and the liquid-vapor interface were measured simultaneously by ADSA-P from sessile solution droplets on FEP-Teflon. Two large proteins (albumin and immunoglobulin G), and four smaller proteins of similar size (lysozyme, ribonuclease, alpha-lactalbumin, and calcium depleted alpha-lactalbumin) were used at varying concentrations. The kinetics of the interfacial tension changes were described using a model accounting for diffusion-controlled adsorption of protein molecules and for conformational changes of already adsorbed molecules, Apart from the interfacial tension changes due to these two subprocesses, the model yields the diffusion relaxation time and the rate constant of the conformational changes. At low concentrations, adsorption of proteins did not always affect the interfacial tension, but its contribution to the decrease in interfacial tension increased with higher bulk concentrations. The decrease due to conformational changes remained a constant value for all proteins. The diffusion relaxation time could not be related to the diffusion coefficient of the protein, probably because of neglect of a reaction component in the model applied. Rate constants for conformational changes were generally lower at the solid-liquid interface, indicating that proteins are more apt to conformational changes at the liquid-vapor interface than at the solid-liquid interface. The least rigid protein, alpha LA(-Ca2+), had the largest rate constant for the conformational change at the two interfaces.