A fluid-wall aerosol flow reactor has been used to thermally dissociate methane to carbon black and hydrogen. The reactor was heated to temperatures in excess of 2000 K by concentrated sunlight. Approximately 90% conversion of methane to hydrogen has been obtained at a reactor wall temperature of 2133 K and an average residence time of 0.01 s. The carbon black produced by reaction is ash free, amorphous, and has primary particle sizes of 20-40 nm. The results of an energy balance as well as conclusions from studies designed to evaluate the effect of co-feeding carbon black particles into the reactor are discussed. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.