AbstractThree general types of commercial particle sizing instruments have been tested with redispersed fly ash and devolatilised coal char in the size range 0.5 to 5 μm. Results are compared with volume‐equivalent diameters obtained by Coulter Counter. Single‐particle light‐scattering counters are found to oversize fly ash, primarily because of the irregular particle shape, but generally to undersize the strongly absorbing char particles. The Malvern sizer, based on Fraunhofer diffraction analysis, grossly oversizes fly ash dispersed in water, probably because most of the particles are too small for the Fraunhofer approximation to be valid; results for carbon char are closer to the expected values, although results for particles dispersed in air and in water show some disagreement. The Aerodynamic Particle Sizer undersizes irregular particles substantially, probably because of increased drag resulting from the irregular shape and tumbling motion of these par