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The effect of hydrostatic pressure up to 105.2 MPa on the phase diagram (temperature vs ratio of volume) of N-isopropylacrylamide gel and its compressibility near the volume phase transition temperature were examined. The volume phase transition changed from a slightly discontinuous one to a continuous one with increasing pressure, and there seemed to be little change of the transition temperature. The compressibility drastically changed from positive values (swollen phase) to negative values (shrunken phase) at the transition temperature. The volume of the gel for the shrunken phase increased in spite of applying pressure in the vicinity of the transition temperature. The obtained anomalous compressibility was analyzed according to the theory of free energy of the gel.

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