Positron annihilation lifetime spectroscopy was used to characterize the reentrant volume-phase-transition behavior of poly(N-isopropyl acrylamide) hydrogel in an ethanol/water mixed solvent. The polymer gel was synthesized with gamma irradiation. The ortho-positronium lifetime (tau(3)) in the gel slowly increased with an increase in the ethanol content in the mixed solvent. tau(3) was not influenced by the volume phase transition. The ortho-positronium intensity decreased with the collapse of the gel in an approximately 10% ethanol/water mixture. When swelled in pure ethanol, tau(3) initially increased with the solvent amount in the gel, showing the destruction of intramolecular hydrogen bonding and the relaxation of polymer chains. The lower critical solution temperature of the gel in the 10% ethanol/water mixture was lower than that in pure water, and tau(3) for various solvent contents showed behavior similar to that seen in pure solvent.