The results of a series of experiments where Electrical Impedance Tomography (EIT) was used to investigate the distribution of solids inside a hydrocyclone is described. A case study is presented for a 44-mm diameter hydrocyclone. Among the characteristics examined are: (i) identification of particle distribution inside the separator, (ii) air core formation as a function of the feed rate and solids concentration, and (iii) the manner in which the air core behaviour can be related to the type of underflow discharge (spray, rape).