The volume phase transition and the microstructure of physically cross-linked hydrogels composed of either N,N-dimethylacrylamide (DMA) or N-isopropylacrylamide (NIPA) and 2-(N-ethylfluoro-octanesulfonamido)ethyl acrylate, FOSA, were studied using differential scanning calorimetry (DSC), water swelling measurements (WS), and small-angle X-ray scattering (SAXS). DSC and WS measurements showed that FOSA/NIPA gels exhibited a volume phase transition (VPT) but that the FOSA/DMA gels did not. The temperature of the VPT (T-VPT) for the FOSA/NIPA gels was lower than that for the covalent NIPA gel, and T-VPT decreased and the transition broadened with increasing FOSA concentration. A peak in the SAXS structure factor indicated that the hydrophobic interactions promoted nanophase separation in both gels-in the dry and hydrated states.