An investigation of the preferential interaction of calcium ions with oxygen atoms in poly(2-hydroxyethyl methacrylate) (PHEMA)-based hydrogels has been carried out. The formation of polymer-Ca complexes was achieved by exposing powdered or fully hydrated samples with 5 mM, 0.1-0.5 M, or saturated CaCl2 solutions for certain periods of time. The characteristics of the polymer-Ca complexes were deduced from the effect of the solute on the equilibrium water content, and from NMR, atomic absorption and infrared spectroscopies. The absence of significant changes in the NMR chemical shift and infrared vibrational wavenumbers for the various functional groups confirmed that polymer complexation with Ca2+ ions involves only weak interactions, possibly electrostatic or ion-dipole interactions. Among the three types of oxygen atoms in PHEMA, hydroxyl oxygen atoms seem to be the most sensitive to the presence of Ca2+ ions. Complexation at the ester oxygen atoms was also evidenced by a new band in the infrared spectra at 1,550 cm(-1). On the other hand, there were no indications that the hydrophobic domains in the backbone and the methyl groups at the side chain of PHEMA interact significantly with Ca2+ ions.