The pyrolysis of CH4 to H-2 and carbon fibers over MCM-41 supported Ni catalyst has been investigated at different temperatures, The initial conversion of CH4 at 873 K is higher than it is at 773 K, but the deactivation at 873 K occurs rapidly. The investigations with SEM and TEM techniques indicate that the pyrolysis of CH4 generates mainly solid carbon fibers at 773 K but mostly hollow ones at 873 K on Ni/MCM-41. Ni metal particles have hardly been found on the tip of or inside the carbon fibers. Raman spectroscopic studies suggest that the carbon fibers formed at 873 K are more graphitic than those obtained at 773 K. The temperature-programmed reactions with O-2 (TPO) and H-2 (TPH) reveal that the solid carbon fibers formed at 773 K show higher reactivity towards O-2 and H-2 than the hollow ones formed at 873 K. (C) 2002 Elsevier Science Inc. All rights reserved.