Carbon microfibers were synthesized from hydrogen-diluted methane over an iron catalyst and characterized by scanning electron microscopy, thermogravimetry (TG), X-ray diffraction (XRD), and Raman spectroscopy. The conversion of methane and the overall selectivity of carbon fiber formation were derived reproducibly as a function of the growth conditions. Blank experiments and experiments with different iron loadings helped to identify the catalytic methane decomposition at about 1270 K, whereas the pyrolysis of methane dominated at higher temperatures leading to a core-and-shell structure of the fibers. Prolonged growth using a CH4/H-2 = 30/70 mixture at 1423 K for 8 h resulted in a diameter of more than 20 mum and a length approaching a limit of about 40 mm. The growth mode was found to depend strongly on the gas-phase composition and the temperature. By lowering the methane content to 10%, or by lowering the temperature to 1348 K, nanofibers were obtained. When diluting methane with helium instead of hydrogen, significantly shorter fibers were formed. The degree of structural order detected by Raman spectroscopy was hardly influenced by prolonged growth at 1423 K, whereas the interlayer distance was found to decrease slightly to 345 pm after 8 h correlated with an increasing thermal stability in air up to 1024 K (50% weight loss). (C) 2004 Elsevier B.V. All rights reserved.