The protonation of polymethacrylic acid (W similar to 230 000) has been studied potentiometrically [(H+)-glass electrode] in different aqueous salt solutions (NaCl, KCl, Et4NI; 0 less than or equal to I less than or equal to 0.5 mol dm(-3)) at 25 degrees C. Measurements have been performed also for polymethacrylic acid (PMA)-CaCl2 mixtures (PMA/Ca2+ = 0.5-3) at I = 0.1 mol dm(-3) (NaCl). The potentiometric data have been analyzed first by using the three-parameters model of Hogfeldt for apparent protonation constants, and then differences in different media have been interpreted in terms of complex formation, Formation constants are reported for Na+, K+ and Ca2+ complexes.