The smoke concentration within the primary zone of an experimental combustor operating at pressures up to 20 atmospheres was determined with a technique using spectral radiance measurements. Exhaust smoke concentration was obtained from reflectance readings of stained filter paper. The primary zone smoke concentration was of the order of 100 to 1000 times greater than at the exhaust. The smoke oxidation rate obtained is compared with reported values for coal particles and seeded laboratory flames. Primary zone smoke concentration was shown to be related to exhaust smoke number and total radiance.