Light scattering was used to study the properties of poly (methacrylic acid), PMA, and its sodium salt, NaPMA, in aqueous solutions. The polymer is an atactic one, composed of around 45% heterotactic and 55% syndiotactic triads. The weight-average molar mass of a single PMA chain (M-w,M-0 = 131 kg/mol) was estimated from experiments with NaPMA in 0.1 M NaCl. From the scattering data at zero-angle it was concluded that PMA is intermolecularly associated in 0.1 M HCl. The data were treated according to the Open Association Model of Elias, which yielded M. for the associated PMA form equal to 188 and 170 kg/mol at 25 degreesC and 55 degreesC, respectively. Furthermore, the second virial coefficient and a relatively high value for the apparent association constant were obtained: A(2) = 1.37 x 10(-3) and 1.24 x 10(-3) mol ml/g(2) and K-app = 3.4 x 10(5) and 2.0 x 10(5) at 25 degreesC and 55 degreesC, respectively. The radius of gyration of NaPMA in 0.1 M NaCl is R-g = 47 nm, whereas for PMA in 0.1 M HCl it is equal to 20.4 nm at 25 degreesC. An increase in temperature from 25 degreesC to 55 degreesC brings about a decrease in R-g to 16.9 nm for PMA in 0.1 M HCl. The changes in thermodynamic functions point to a hydrophobically driven association process of PMA chains.