Uniform monodisperse FeOOH and hematite (Fe2O3) nanoparticles were synthesized at room temperature via a facile sonochemical reaction between FeCl3 and NaOH. Fe2O3 nanoparticles were then added to acrylonitrilebutadiene-styrene (ABS) polymer. Nanoparticles and nanocomposites were characterized using X-ray diffraction, scanning electron microscopy, Fourier transform infrared, and photoluminescence (PL) spectroscopy techniques. The effect of the presence of FeOOH and Fe2O3 nanoparticles in the polymer matrix on the thermal properties was studied using thermogravimetric analysis and derivative thermogravimetric analysis, in both air and nitrogen atmospheres. The magnetic properties of the samples were also investigated using an alternating gradient force magnetometer. We found that the Fe2O3 nanoparticles exhibit a ferromagnetic behavior with a saturation magnetization of 0.869 emu/g and a coercivity of 191 Oe at room temperature. The coercivity of ABS/ Fe2O3 (5%) nanocomposites is found to be 252 Oe, higher than 226 Oe which is obtained for ABS/Fe2O3 (20%). (C) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012