An adsorption model is proposed which assumes a different partial molar surface omega for adsorbed molecules depending on the degree of saturation of the adsorption layer. Equations for an adsorption isotherm and a surface tension isotherm are derived for the adsorption of one single surfactant or of a mixture of surfactants. The model assumes that the molecules may adsorb in two different states, i.e. a large omega at smaller surface pressures Pi and a smaller omega at large surface pressures Pi. Adsorption data of (N-n-hexadecyl-N,N-dimethylammonio)acetic acid bromide at the air/solution interface are used to demonstrate the application of the theory. The experimental results are in very good agreement with the model.