In this research, the synthesis and properties of thermally responsive complex polymer networks containing Fe3O4 nanoparticles were studied. First, a stable ferrofluid containing Fe3O4 nanoparticles was synthesized via a coprecipitation method in the presence of a poly(acrylic acid) oligomer. This stable ferrofluid could mix well with water-soluble monomers by the adjustment of its pH value. Second, a thermally responsive copolymer was synthesized in the presence of the ferrofluid containing Fe3O4 nanoparticles to obtain the complex polymer networks. By the adjustment of the pH value, the ferrofluid could remain stable in the polymerization system, in which N-isopropylacrylamide (NIPAAm) and methacrylic acid (MAA) were used as comonomers to provide thermoresponsive properties and acid groups and ammonium persulfate and sodium metabisulfite were used as the redox initiator system. Several variables, such as the molar ratio of MAA to NIPAAm, the concentrations of the monomers and crosslinking agent, the addition of an ammonium solution, and the content of the ferrofluid, were studied in this polymerization. Their effects on the morphology, structure, polymerization rate, and thermal properties of the complex polymer networks were discussed. The swelling and thermoresponsive behaviors of the complex polymer networks containing Fe3O4 nanoparticles were also studied, and the composition-morphology-property relationship was established. (c) 2005 Wiley Periodicals, Inc.