The concept of using the relative density of a fluid to effect separation in mineral processing operations is an ancient one. Towards the middle of the last century, the Dutch State Mines (DSM) in the Netherlands accidentally discovered that separation in hydrocyclones could occur not only by size but also by density. They patented the concept and the design, which has since developed into what is known today as the dense-medium cyclone (DMC). Typical dense media used in mineral processing operations are suspensions of magnetite or ferrosilicon particles in water. Being denser than water, these particles render the medium unstable. Literature shows that medium behaviour is process determining. Until now the behaviour of the heavy medium in a DMC had not been quantified. This thesis develops a method for exploring medium behaviour in more detail. The DMC being a closed container, any investigation should not disturb the flowing fluid in it. Computed tomography using gamma radiation offered itself as a potential candidate towards a non-invasive method to study the density gradient of the medium in the DMC. Hundertmark's (1965) pioneering efforts in this direction stopped short of tomographic treatment of the data. To better understand the behaviour of a heavy medium in a DMC, a 137 Cs isotope was used as the radiation source and a series of scans were carried out on a 350-mmfibreglass DMC, with a magnetite suspension as the dense medium. Two methods of reconstructing the density gradients from these scans were tried — the filtered back-projection (FBP) technique and the piecewise linear method (PLM). For the first time, tomograms were generated to visualise the radial and axial density gradients. Medium segregation was found to be unaffected by changes in feed pressure and feed medium density. The air core and its characteristic 'corkscrew' shape were also mapped. The tomograms also revealed that the density of the medium close to the cyclone wall was lower. Computed tomography seems to hold tremendous potential in unearthing the mysteries of the DMC and could well lead to a fundamental model and/or an improved design of the DMC.