The development and application of electrical resistance tomography for on-line auditing of an industrial hydrocyclone separation is reported. The work demonstrates the retrofitting of electrodes into a commercially available separator and their use in laboratory, pilot plant and plant scale investigations of clay refining. A number of new and significant applications are described including: the development of methodologies to allow observation of the occurrence of faults in underflow discharge (spraying, roping, blockage); accurate measurement of the air core size for different operational conditions; direct calculation of solid concentration profiles based on parametric reconstruction of conductivity data in three dimensions.