Temperature (T) induced nanoparticle formation of isotactic (iPMA) and atactic (aPMA) poly(methacrylic acid) chains is investigated in aqueous solutions in the presence of divalent cations (Mg2+ and Ca2+) by UV spectroscopy, visual observations and pyrene fluorimetry. In aPMA solutions, aggregation and phase separation take place with increasing T. The onset of aPMA aggregation (heating) and re-dissolution of the aggregates (cooling) is shifted to lower Ts with increasing ionic strength of the solutions. iPMA associates partly decompose upon heating, but stable nano-sized particles are present at all Ts. Structural characterization of the aggregates was carried out by dynamic and static light scattering measurements at 25 °C. Results revealed that aggregates of both PMAs with Mg2+ ions involve a lot of water and have a rather even mass distribution. This is attributed to strong hydration of Mg2+ ions and their preference towards monodentate binding to carboxylate groups. Differently, Ca2+ ions bind more strongly and in a bidentate manner, they lose the hydration water and form aggregates with several structural elements, depending on chain tacticity.