Swelling behavior of poly(alpha-hydroxy acrylic acid) (PHA) gel was investigated in water/organic solvent mixtures (acetone, dioxane, dimethyl sulfoxide: DMSO) in comparison with poly(acrylic acid) (PAA) gel. The swelling degrees of PHA and PAA gels significantly decreased with increasing acetone or dioxane content in the mixed solvents, while the deswelling in aqueous DMSO was less apparent. The deswelling region was dependent on the dissociation state of the carboxyl group; when the carboxyl group in the gels was dissociated (-COO-), significant deswelling occurred in 30-60 vol.% of acetone or dioxane for PHA gel and in 40-80 vol.% for PAA gel. When the carboxyl group was in an acid form (-COOH) or only slightly dissociated, the deswelling region was 0-30 vol.% for PHA and 80-100 vol.% for PAA. The unique swelling behavior of PHA gel was ascribed to intermolecular hydrogen bonding. Effects of dielectric constant on the swelling are also discussed.