A large quartz reactor (97 cm x 4.2 cm) was constructed to study the decomposition of methane in a fluidized bed of carbon-black particles, which acts as a catalyst. Experiments were run for 400-2000 min using three types of carbon black, temperatures of 810-980 degrees C, and space velocities of 95-2 10 h(-1). Under optimum conditions, the decomposition produces >= 40% hydrogen in the reactor effluent, solid carbon that remains on the carbon bed, and little else. However, at the higher temperatures 965 degrees C, gas-phase decomposition occurred with formation of polycyclic aromatic hydrocarbons and suspended carbon particles. Typically, hydrogen production decreased during the first 50 min of an experiment, then remained approximately constant for an extended period and finally fell to a low rate by 1600-2000 min. Various measurements were made on the carbon at the beginning and end of the experiments in an attempt to understand the decrease in hydrogen production. (c) 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.