The paper discusses the experience gained and the results of experiments performed whilst endeavouring to obtain satisfactory performance from electrostatic precipitator plant.The principal factors determining drift velocity in precipitators are discussed. A simple approximate relation for the field strength in a parallel-plate horizontal-flow precipitator is developed, based on that generally accepted for a wire-in-cylinder precipitator unit.Comprehensive expressions are introduced for calculating the theoretical performance of precipitators under conditions of nonuniform gas flow and with dust of mixed composition. It is shown that individual drift velocities for different particles may be determined from appropriate experimental observations.The suggestion that pulverized fuel dust is composed largely of two distinct types of particles, one wholly ash and the other coal minus volatiles, i.e. coke, is discussed. Evidence, principally from South African sources, is put forward to support this suggestion.The effective drift velocities obtained in practice for plants operating on Natal duff coal are calculated for a given field strength and are compared with those measured elsewhere. Separate drift velocities for average ash and coke particles are given, and the former are shown to have higher useful drift speeds than the latter.It is shown that, as predicted in theory and as previously proved under laboratory conditions, the drift velocities of particles of the sizes encountered in practice do, in fact, increase with the particle diameter, given a constant composition.Finally, notes are included on the interpretation of test data in order to locate and correct the more common sources of poor performance of precipitators.