Solid-liquid filtration monitoring by means of a single inexpensive 16-element ring sensor array for electrical resistance tomography (ERT) is described. The high sensitivity of the UMIST Mark 1b-E data acquisition system led to the unexpected finding that this tomography array can detect movement of the liquid level during filtration. This degree of sensitivity was also capable of detecting any tilt of the filter assembly, so that pathological behaviour due to malfunction or accidental displacement of the filter support plate could also be identified. Moreover, illustrative distortions of a filter cake formed by artificial surface depressions were readily observed in tomograms reconstructed by linear back projection. It is concluded that ERT has great potential for instrumenting and detecting ＇pathological＇ behaviour of filtration processes in the pharmaceutical and fine chemicals industries. (C)2000 Elsevier Science S.A. All rights reserved.