The design of radiation heat exchangers in the high-temperature range is influenced by the solid loading of the participating media. In order to obtain data and criteria for a more sophisticated design, emission measurements of different gas-dispersed solids (quartz sand, ash from fluidized bed combustion, corundum) have been carried out in an experimental plant specially designed for this purpose. An equation is derived to describe the particle emission coefficient in a geometry with surrounding grey walls. The experimental data fit this model well. An examination of the composition of the solids has shown the considerable influence of the quartz content on the radiation behaviour.