Three ionic and nonionic temperature-sensitive hydrogels, poly(N-isopropylacrylamide) (PNIPAAm), poly(N-isopropylacrylamide-co-acrylic acid) (NIPAAm-AA), and poly(N-isopropylacrylamide -co-hydroxypropyl methacrylate) (NIPAAm-HPMA), are synthesized and used to concentrate the lignin solutions of interest in the pulp and paper industry. The interactions between hydrogels and solutes play an important role in the separation. Because of the absorption of lignins in the hydrogels, the separation efficiency of the PNIPAAm hydrogel is low. By introducing the hydrophilic monomers AA and HPMA into the NIPAAm hydrogel, the separation efficiencies are increased because of the ionic repulsion between lignin and the incorporated hydrophilic groups. In addition, the separation efficiencies are affected by the extent of cross-linking and the solute concentration. The time dependence of the separation efficiency suggests that the separation is diffusion-controlled.