Evidence from dynamic viscoelasticity measurements has been found for reversible network formation via the physical association of hydrophobic side-groups during phase separation at around 33 degrees C in a 1.22 wt.% aqueous solution of poly(hi-isopropyl acrylamide) (PNIPA) having a molecular weight of 2.1 x 10(6). The frequency omega dependence of the dynamic storage and loss moduli G＇ and G " showed homogeneous fluid behaviour at 25 degrees C, while at 35 degrees C, a plateau zone was found in the G＇ versus omega curve and G＇ was larger than G ", indicating the formation of a network. The increase in G＇ was instantaneous at the phase separation temperature and was not a progressive process.