An adsorption model is proposed that assumes different partial molar surface areas 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., large at smaller surface pressures Pi, and smaller at large surface pressures Pi. Adsorption data of N-n-hexadecyl-N,N-dimethylammonio acetic acid bromide at the solution/air interface are used to demonstrate the application of the theory. Experimental results are in very good agreement with the model.