AbstractThe preferential solvation of poly(methacrylic acid) and poly(sodium methacrylate) in waterdioxane mixtures was investigated by means of specific refractive index increment measurements on dialysed and undialysed solutions. The correlation with the solvent‐induced conformational transition of poly(methacrylic acid) was also studied. Potentiometric titrations indicate that this transition occurs at around 25–30 vol.‐% of dioxane. Comparison with previous results on poly(acrylic acid) and poly(sodium acrylate) in the same solvent system and on poly(methacrylic acid) in water‐2‐chloroethanol mixtures, shows differences in the mode of solvation of the compact conformation. It is suggested that this may be dependent on the existence of water‐organic solvent specific interactions such as the formation of a water‐dioxane hydrogen