This paper presents a retention model for proteins on an reversed-phase chro-matography support in which retention is a function of the number (Z) of solven molecules required to displace the solutte from the surface. An equation is dervied that relates the capacity factor of a protein to the displacing agent concentration and the stoichimonetry of solvent-solute displacement. Experimental tests of the model indicate that each protein has a unique Z value and that Z is directly proportional to the molecular weight of a series of protins when 60%formic acid is used as the mobile phase additive. This relationship is polypeptide solutes and the support. De-sorption curves for proteins also become more convex with incerasingly molecular weight, as predicted by the retention model. In the solvent series of methanol, etha-nol, propanol, the Z number decreases form the C1 to C3 alcohol. The Z number for any particular solvent is also related to other mobile phase additives, such as acids, and the concentration of additives.