Caption of Hg(II) from acid aqueous solution by immersed poly(N-vinylimidazole) hydrogel particles was studied as a function of pH, counterion, and cation concentration. Fitting parameters to several sorption isotherms have been determined. Their values depend mostly on pH and less, on temperature and counterion, and suggest a large affinity of imidazole groups in the gel and mercury cations. Practically total removal (94.4%) of Hg(II) is achieved at pH = 2, with 10 g of dry gel per liter of solution, when cation concentration was as large as 15,000 ppm (0.075 M). Polymer protonation decreases about fourfold the cation affinity, supporting competitive protonation-complexation mechanisms. By its side, metal uptake decreases polymer protonation. Thermal stability of loaded gels decreases with respect to metal free hydrogels. Scanning electron micrographs reveal no changes in the gel morphology upon cation binding, but T-g increases significantly with the Hg(II) content of loaded gels and swelling decreases moderately, indicating the role of the cation as ionic crosslinker. Practically total elution of Hg(II) is achieved with 1 M HNO3 in consecutive loading-elution cycles.