One-pot synthesis of thermoresponsive magnetic composite microspheres with a poly(N-isopropylacrylamide) (PNIPAM) shell and a Fe3O4 core is demonstrated. Temperature sensitivity of PNIPAM was adopted to design the novel synthesis pathway. The as-prepared composite microspheres have an obvious core-shell structure with a mean size of approximately 250 nm. The Fe3O4 core is approximately 5 nm and the thickness of the PNIPAM shell is approximately 10 nm. The content of Fe3O4 in the composite microspheres can be controlled by this method. The composite microspheres experience a swelling and shrinking process in water by adjusting the temperature below and above the lower critical solution temperature (LCST) around 32 degrees C. These microspheres also show fine response to an external magnetic field. This work presents a platform to synthesize organic/inorganic composite microspheres in a facile and efficient approach. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 27022708