The solubility and association of N-(2-hydroxypropyl)methacrylamide (HPMA) copolymers containing N-(2-hydroxyethyl)piperazine-N’-ethanesulfonic acid (HEPES) side chains in aqueous solvents were investigated using light scattering methods. The weight-average molar masses, diffusion coefficients, and hydrodynamic sizes of the copolymer molecules and their aggregates were studied as a function of HEPES content, solvent pH, and copolymer concentration. The copolymers with a high content of HEPES-containing side chains (60-100 mol %) associated in aqueous buffers, forming multichain aggregates (clusters). The formation of these aggregates was strongly dependent on solvent pH. Well-defined aggregates were observed at pH values between 6 and 7 (the hydrodynamic radius, R(h), was about 115 nm), and a broad distribution of large aggregates (R(h) almost-equal-to 250 nm) was found at pH 8 and 10. These aggregates disappeared completely at pH 3. The observed effect of solvent pH on the solution properties of copolymers was explained qualitatively by the well-known electrolyte behavior of HEPES molecules. The aggregation found at solvent pH between 6 and 7 was related to the zwitterionic structure of HEPES-containing side chains which is formed in the vicinity of the half-neutralization point of the copolymers (pH approximately 6.5). The attractive electrostatic forces among ion pairs (dipoles) of zwitterionic HEPES moieties appeared to be responsible for both the intermolecular and intramolecular association.