LW Nicholf, W. J. H. Jacksonf, and D. J. WinzorJ abstract: Previously established concepts of multiple equilibria have been applied to a system in which a protein monomer coexists with a single higher polymer and both bind a low molecular weight solute (S). The treatment permits the number of binding sites permole of monomer and polymer to assume any integral values (including zero), but assumes that sites within a mo-lecular entity are equivalent with regard to binding, ex-cept for a statistical effect. An equation is derived which relates the free equilibrium concentration of S in any equilibrium mixture to a binding function, r, defined on a weight-concentration scale as the concentration of S bound divided bythe initial protein concentration. It is shown that equations of the same form describe binding in other types of systems, including those involving isomerization of the protein and a system in which a series of polymers coexist in equilibrium. In the special case where no binding sites are lost in successive polymeriza-tion reactions and the intrinsic binding constants are