The kinetics of the thermal decomposition of CH4 for co-producing H-2 and carbon black are examined for a vortex flow confined to a cavity receiver and directly exposed to mean radiative power fluxes of 2500 kW/m(2). The CH4 flow is laden with carbon particles that serve as radiant absorbers and nucleation sites. The temperature dependence of the reaction rate can be described by the Arrhenius law with an apparent activation energy in the range 147-162 kJ/mol and a frequency factor in the range 1.07 x 10(6)-7.54 x 10(6) s(-1), assuming a first-order decay rate law and either a mixed flow or a plug flow reactor model. Based on the latter model, a residence time of 0.3 s is required for obtaining 90% chemical conversion at 1500 K. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.