Nano-microstructured porous carbon composite fibers (Fe2O3@C/FeO@C/Fe@C) were synthesized by the thermal decomposition of ferrum alginate fibers. The ferrum alginate fiber precursors were prepared by wet spinning, and calcined at 3001000 degrees C in high purity nitrogen. The resulting composite fibers consist of carbon coated Fe2O3/FeO/Fe nanoparticles and porous carbon fibers. All the prepared nanostructures were investigated using thermal gravimetry, X-ray diffraction (XRD), Fourier transform infrared spectroscopy, transmission electron microscope (TEM), and nitrogen adsorptiondesorption isotherm. The results show that there are five stages in the decomposition process of the ferrum alginate fibers. Transitions between the five stages are affected by the decomposition temperature. XRD results show that maghemite (Fe2O3), wustite (FeO), martensite (Fe) nanoparticles were formed at 300500 degrees C, 600700 degrees C, 8001000 degrees C, respectively. Scanning electron microscopy and TEM results indicate that the composite fibers consist of nanoparticles and porous carbon. The diameter of the nanosized particles increased from 100 to 500 nm with increasing reaction temperature. The nitrogen adsorptiondesorption results also show that the composite fibers have a micro- and mesoporous structure. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013