When investigated under high concentration conditions, hyaluronan (HA) solutions in physiological saline are shown to generate stable superstructures. An abrupt change in the rheological properties observed on increasing the temperature suggests the breaking of certain cooperative bonds. The thermal disruption of the HA superstructure is accompanied by a sharp transition from a long- to a restricted-connectivity water structuring, which is interpreted as a concurrent transition from a stable to a temporary polymer network. The intermolecular associations are considered to be originated by hydrophobic interactions between the nonpolar groups of the polymer backbones.