A new isotherm is presented for adsorption of proteins, the available area isotherm. This isotherm has a steric basis, unlike the (steric) mass action model. The shape of the available area isotherm is determined only by geometric exclusion. With the new isotherm, experimental results can be fitted equally well as with existing ones, such as the steric mass action model. However, it has several advantages. For fitting of single-protein isotherms one less parameter is needed, its theoretical basis is more realistic, it can be extended consistently to many components, it is applicable to adsorption by both ion-exchange and hydrophobic interaction, and it can easily be combined with equations describing the effect of pH and ionic strength to obtain a complete description of adsorption behavior. (C) 2004 American Institute of Chemical Engineers.