Unfolding of proteins has often been mentioned as an important factor during the adsorption process at air-water interfaces and in the increase of surface pressure at later stages of the adsorption process. This work focuses on the question whether the folding state of the adsorbed protein depends on the rate of adsorption to the interface, which can be controlled by bulk concentration. Therefore, the adsorption of proteins with varying structural stabilities at several protein concentrations was studied using ellipsometry and surface tensiometry. For beta-lactoglobulin the adsorbed amount (Gamma) needed to reach a certain surface pressure (Pi) decreased with decreasing bulk concentration. Ovalbumin showed no such dependence. To verify whether this difference in behavior is caused by the difference in structural stability, similar experiments were performed with cytochrome c and a destabilized variant of this protein. Both proteins showed identical Pi-Gamma, and no dependence on bulk concentration. From this work it was concluded that unfolding will only take place if the kinetics of adsorption is similar or slower than the kinetics of unfolding. The latter depends on the activation energy of unfolding (which is in the order of 100-300 kJ/mol), rather than the free energy of unfolding (typically 10-50 kJ/mol). (c) 2006 Elsevier Inc. All rights reserved.