Measurement of ascending and descending breakthrough curves of two proteins, hen egg white lysozyme and bovine serum albumin, on commercially available hydrophobic-interaction chromatographic columns reveal that only one part, i.e., the reversible adsorbed fraction, of the total bound amount can be desorbed by dilution of the feed solution. The other part is bound irreversibly; its binding behavior follows a hysteresis. Our procedure allows a clear separation of both parts. The reversibly adsorbed fraction of lysozyme can be represented with a Henry-type isotherm. For both proteins and for the reversible as well as the irreversible part, the "adsorbed" amount increases when the number of charges of the proteins decreases by approaching the isoelectric point.