The sigmoidal nature of circular dichroism (CD) changes for hyaluronic acid solutions as a function of solvent composition or temperature is studied as a function of chain length by using oligomers. We find a chain length effect with approximately nine disaccharides required for the structural transition as a function of organic solvent, which proves that the transition is cooperative with large transition enthalpy and entropy. The transition also depends on sample concentration as expected for strand association, and this was investigated in detail for oligomers 12 and 16 disaccharides long. Indeed, it was possible to prevent completely the transition in mixed solvent with sufficient dilution of these oligomers, which demonstrates strand association. The CD data in mixed solvent as a function of oligomer concentration were fit with various models for association of two and more strands. Simplex methods were used to investigate the vector space of unknowns for the models, and two-strand models were shown to consistently give a better fit. A cooperative two-strand zipper model which allows relative sliding of the chains had the smallest fitting error and produced the following thermodynamic parameters (in terms of a duplex of disaccharide units) for the ordered structure in an aqueous solution containing 45% v/v ethanol, 12.5 mM NaH2PO4, and 7.5 mM H3PO4: enthalpy of growth, -1.0 +/- 0.3 kcal mol-1; entropy of growth, -2.3 +/- 1.3 eu mol-1; enthalpy of initiation, -20 +/- 3 kcal mol-1; entropy of initiation, -71 +/- 15 eu mol-1. The results are consistent with a double-stranded and helical structure for hyaluronic acid in solutions of reduced dielectric constant.