Volumetric absorption of radiation in a solar reactor can be achieved by seeding gas in the reaction chamber with a cloud of fine radiation absorbing particles. Previous studies indicated that very high absorption effectiveness can be attained if the seed particle linear dimension is smaller than the characteristic absorption length for the light passing through the particle suspension. It has been confirmed by experiment that for sub-micron particles a seeding mass loading of the order of 0.1 g carbon black powder per cubic meter of irradiated gas is sufficient for attainment of adequate radiation absorption. The development of especial equipment by which gas seeding with sub-micron particle powder can be executed and monitored in a simple and precise manner is described. The performance of the seeding device was verified experimentally and the results are here related. (C) 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.