This study addresses the solar thermal cracking of methane for the co-production of hydrogen and carbon black as a medium to avoid CO2 emissions from natural gas combustion processes. The objective of this work is to numerically simulate the transport processes of momentum heat and mass in an indirect heating solar reactor, which is fed with an argon-methane mixture. The reactor is composed of a cubic cavity receiver, which absorbs concentrated solar irradiation through a quartz window and a graphite reaction tube is settled vertically inside this cavity. A series of numerical experiments were carried out in order to gain a better understanding of the interaction between the several transport phenomena taking place. The simulations showed that, in general, when the temperature of the reaction chamber is higher than 2000 K, the methane conversion is practically 100%. To validate our simulation results we compared them with available experimental data obtaining good agreement. Moreover, our results clearly evidence that most of the reaction takes place at the bottom of the reactor, which is the zone with the highest temperature profiles. Therefore, we propose modifications in the reactor design to increase conversion. The results of this work can thus serve to improve design and control of solar reactors for light hydrocarbons. Copyright (C) 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.