In this research, the thermoresponsive composite latex particles were prepared via W/O miniemulsion polymerization. Fe3O4 nanoparticles were homogeneously dispersed inside the poly(NIPAAm-co-MAA) latex particles. In the first step, PAA oligomers were used as stabilizers to produce a stable water-based Fe3O4 ferrofluid, which could mix well with the water-soluble monomers. In the second step, the Fe3O4/poly(NIPAAm-co-MAA) composite latex particles were synthesized via W/O miniemulsion polymerization. This polymerization proceeded in cyclohexane at room temperature, with Span80 as the emulsifier, NIPAAm as the thermoresponsive monomer, MAA as a comonomer with -COOH functional groups, and APS/SMBS as the redox initiator system. The distribution of Fe3O4 nanoparticles inside the composite latex particles was expected to be homogeneous. The nucleation and morphology of the composite latex particles were mainly controlled by the concentration of the surfactant, Span80, in cyclohexane. The properties of the composite latex were examined with several instruments such as DSC and TGA. Finally, the superparamagnetic and thermoresponsive characteristics of this functional composite latex were also investigated. (c) 2006 Wiley Periodicals, Inc.