Abstract The dissociation-association behavior of phycocyanin has been studied by sedimentation-equilibrium measurements under various pH and ionic-strength conditions. The weight-average molecular weight of phycocyanin changed according to the changes in concentration of the solute. From the concentration dependence of the molecular weight, it has been claimed that the dissociation-association equilibrium system of phycocyanin is trimer\ightleftharpoonsmonomer at pH 6.8 in the relatively low concentration region and hexamer\ightleftharpoonsmonomer at pH 5.4. However, the coexistence of the trimer\ightleftharpoonsmonomer and hexamer\ightleftharpoonsmonomer systems is in evidence in the concentration region larger than 0.06 g/dl. This dissociation-association behavior of phycocyanin is supported by the experimental results obtained from sedimentation-transportation and osmotic-pressure measurements.