The volume change, &UDelta; V-h, accompanying the hydrophobic hydration associated with the volume phase transition in Poly(N-isopropylacrylamide) gels was measured by a simple method. The hydration accompanies a negative &UDelta; V-h-2.5 cm(3)/mol. The P-T phase diagram, the coexistence Curve, for the gels was determined from the swelling ratio-pressure curves up to 350 MPa for various constant temperatures. The contour of the coexistence curve is shaped like an ellipsoid on the P-T plain, which is a feature peculiar to the reversible pressure-temperature denaturation cf a protein. The thermodynamic analysis of the Clausius-Clipeyron relation for the measured &UDelta; V-h elucidates that the obtained coexistence curve represents the phase boundary between thermodynamic different phases like the two phases, native and denatured, of a protein and gives the transition enthalpy, &UDelta; H, 5.2kJ/mol by estimate, which well coincides with the transition heat measured by a calorimetric method. Considering the volume-dependent free energy, &UDelta;ν(mi) • P, for the mixing free energy of the gel, we can fit the calculated curve to the measured swelling ratio-pressure Curve of PNIPA gels. The value of &UDelta;ν(mi) changes the sign from negative to positive above around 100MPa. © 2005 Wiley Periodicals, Inc.