An experimental measurement of the radiative heat flux from a gas-solid suspension to wall was made in a water cooled heat exchanger. The heat-transfer test section was 5 cm ID and 90 cm long. Operating temperature ranged from 200 to 600degreesC and the gas velocity at the inlet of the test section varied from 6 to 11 m/s. The suspension densities covered a range from 3 to 35 kg/m(3). Time-averaged radiative heat flux was directly measured using a radiometer, which allowed to determine suspension emissivity using a measured suspension temperature. Radiative heat flux and suspension emissivity showed strong dependence on the suspension density. The effect of particle size on suspension emissivity was also studied by using two different sizes of particles. Experimentally determined suspension emissivities, which ranged from 0.3 to 0.85, agreed well with the predictable suspension emissivities based on an independent scattering theory. The contribution of radiation to total heat transfer was about 40 - 50% for the operating condition used in this study.