A theoretical basis is presented for the concept of statistical pyrometric particle sizing (SPPS) in fluidised bed combustion (FBC). The theory makes it possible to obtain the size distributions for observable particles within given temperature ranges, and thus a statistical estimate can be made of the dependence of the temperature of a fuel particle on its size during FBC. The SPPS method has been experimentally validated by comparison with an independent reference method in a miniature fluidised bed reactor. Graphite spheres were burned one by one and measurements were taken with a two-colour fiber-optic pyrometer. Pyrometric signals were obtained many times during each particle＇s combustion and SPPS was applied to the results. The generation of CO2 was also recorded and the evolution of the particle＇s size with time was determined from the CO2 mass balance, assuming shrinking core combustion. The average sizes determined by SPPS and also from the CO2 mass balance were in good agreement for all temperature ranges. The elevation of the particle＇s temperature above that of the bed varied between 20 and about 200 degrees C.