When gamma rays are used to charge particulate matter in stack gas, high collection efficiencies are obtained. These high efficiencies are achieved for all sizes of particulates, including those of sub-micron size. The latter represents a significantly different result from that of the conventional Cottrell electrostatic precipitator with its corona ionization rather than gamma-ray ionization. A charge separation theory is suggested as the reason for collecting equal numbers of positively and negatively charged particles. An experimental test rig is described using 10 000 Ci of 60Co, and a conventional stoker-fed furnace.