Europium ions were used as fluorescent probes for examining the interaction of polyanionic hydrogels of chemically cross-linked poly(acrylic acid) and poly(methacrylic acid) with multivalent cations in aqueous medium. The fluorescence data indicate a strong and asymmetric binding of carboxylate groups to Eu3+ ions, leading to the expulsion of up to five water molecules from the solvation shell of the ion. The relaxation studies of the nonradiative energy transfer from europium to neodymium ions inside the gel revealed the formation of aggregates consisting of ca. seven rare earth ions (together with the corresponding countercharges of the network chains).