Osmotic coefficients, phi, and apparent molar volumes, Phi(V), of aqueous solutions of isotactic, syndiotactic, and atactic poly(methacrylic acid), i-PMA, s-PMA, and a-PMA, respectively, were measured at 298 K in dependence on polymer concentration and on degree of neutralization, alpha(N), of carboxyl groups. The solutions of i-PMA have lower phi values than those of a-PMA and s-PMA in the whole region of alpha(N). Molecular dynamics simulation studies of the isotactic and the syndiotactic PMA 101-mer have shown that lower phi is a consequence of a shorter distance between charges, which leads to a greater charge density of the isotactic polymer and thus to stronger binding of counterions. The experimental phi data were analyzed using a cylindrical cell model. Good agreement between theory and experiment was achieved when charges on the polyion were distributed periodically along the z-axis of the polyion in accordance with the simulation results. The alpha(N) dependence of the Phi(V) data pointed out that all PMA isomers bind an appreciable amount of water in the elementary dissociation process (electrostriction). For a- and s-PMA, the Phi(V) values decrease linearly with increasing alpha(N), whereas they show a marked nonlinear dependence in the case of i-PMA for alpha(N) < 0.6. The latter finding was ascribed to a very high intermolecular association tendency of the isotactic polymer. This association tendency of PMA chains was confirmed by light scattering measurements. It is present in both i- and a-PMA solutions but is much more pronounced in the i-PMA case.