An extensive plant-orientated testwork campaign was undertaken to quantify and determine the causes of significant dense medium cyclone operating inefficiencies. Comprehensive plant trials, with 20–30 people sampling up to 100 process (medium and coal) streams simultaneously for eight hour shifts permitted both magnetite media and coal processing characteristics to be simultaneously quantified. Dense medium classification and coal partition curves were principally used to show that the root cause of inefficient cyclone performance was related to incorrect magnetite particle size in the cyclone circuits. When magnetite with the correct specifications was used in the plant, the reasons for coarse magnetite in the cyclone circuits was found not to result from selective fine magnetite losses in the small coal circuits, but from a fundamental design flaw in the magnetite distribution system. The flawed system was shown to prevent correct specification magnetite entering the cyclone circuits, thereby starving these coal circuits of ultrafine magnetite particles. Redesign of the distribution system, which maintained existing sumps and pumps but revised flows and functions, was demonstrated to significantly increase dense medium cyclone separating efficiencies. Further plant testwork results are presented which show that plant feed rate plays a critical role in dense medium cyclone separations, with both separating density and efficiency being sensitive to raw coal feed rate. To extract maximum efficiency from these units, it is critical to control RD50.