Near, mid and far infrared spectra have been recorded for methyl methacrylate-methacrylic acid copolymers and their sodium salts. Methyl methacrylate-methacrylic acid copolymers, with various methacrylic acid contents were prepared by free radical polymerization. The sodium salts of (methyl methacrylate-methacrylic acid) copolymers were prepared by reaction of methacrylic acid units with sodium hydroxide. The acid content in copolymers was estimated by calorimetry and by reactivity ratios of monomers. The results showed that sodium MMA-MAA copolymers prepared by neutralization form ionic clusters and retain in its backbone non-neutralized acid groups. The results showed the effect of composition on spectroscopic data. The absorption/retention of water was observed in the examined samples. (C) 2003 Elsevier Ltd. All rights reserved.