Mammalian small heat shock proteins (sHSP) form polydisperse and dynamic oligomers that undergo equilibrium subunit exchange. Current models of their chaperone activity hypothesize that recognition and binding of protein non-native states involve changes in the oligomeric state. The equivalent thermodynamic representation is a set of three coupled equilibria that includes the sHSP oligomeric equilibrium, the substrate folding equilibrium, and the equilibrium binding between the sHSP and the substrate non-native states. To test this hypothesis and define the binding-competent oligomeric state of human Hsp27, we have perturbed the two former equilibria and quantitatively determined the consequences on binding. The substrate is a set of T4 lysozyme (T4L) mutants that bind under conditions that favor the folded state over the unfolded state by 10(2)-10(4)-fold. The concentration-dependent oligomer equilibrium of Hsp27 was perturbed by mutations that alter the relative stability of two major oligomeric states including phosphorylation-mimicking mutations that result in the dissociation to a small multimer over a wide range of concentrations. Correlation of binding isotherms with size exclusion chromatography analysis of the Hsp27 oligomer equilibrium demonstrates that the multimer is the binding-competent state. Binding occurs through two modes, each characterized by different affinity and number of binding sites, and results in T4L.Hsp27 complexes of different hydrodynamic properties. Mutants of the Hsp27 phosphorylation mimic that reverse the reduction in oligomer size also reduce the extent of T4L binding. Taken together, these results suggest a central role for the oligomeric equilibrium in regulating the chaperone activity of sHSP. The mutants identify sequence features important for modulating this equilibrium.