The adsorption behaviour of the commercially available whey protein isolate, PROTARMOR, at the air-water and oil-water interface has been investigated by two different methods. The adsorption rate at a constant interface area (`isochorous mode`) as a function of time was measured by a method using a `bursting membrane` device. After 10 min the slope of increase in film pressure reaches higher values at the air-water surface compared to adsorption at the oil-water interface. The slope of increase in film pressure at a constant interfacial area is strongly dependent on the protein concentration in the aqueous phase. For pressure-area curves (`isothermal mode`) a rectangular Teflon trough was used with the surface layer being compressed by moving a Teflon barrier. From these measurements it can be concluded that the adsorption of Protarmor at the the air-water surface is reproducible with this method. There is evidence that the adsorption of this protein mixture at the air-water surface is reversible. (C) 1998 Elsevier Science Ltd. All rights reserved