Bacterial oxidation of a sulphide mineral concentrate is a complex exothermic reaction involving gaseous, liquid and solid phases. The conditions within the bacterial oxidation reactor must be maintained in a range where the maximum oxidation rate occurs and the bacterial culture can thrive, The operating conditions that require particular attention when designing the reactor include the temperature and dissolved oxygen level. Other aspects requiring close attention are the homogeneous suspension of the solids, the materials of construction and slurry transfer between reactors. Temperature control may be achieved by contacting cooling water through a conducting surface which is in contact with the mineral slurry. The advantages and limitation of tank wall cooling and immersed cooling coils are examined The air dispersion and the solid suspension duty is performed by a mechanical agitator A number of commercial suppliers have developed agitators which perform these dual functions without damaging the bacterial cells. The relative merits of these specialised agitator designs are discussed. Correct selection of the materials of construction for all parts of the reactor in contact with the slurry is vital because of the extremely corrosive bacterial oxidation solution and the abrasive nature of the mineral particles. The materials used in existing plants are reviewed together with their performance. Conventional tank launder systems for slurry transfer are often unsuitable because of the relatively low slurry density, low flow rates and the high specific gravity of the sulphide concentrates. A number of alternative methods for transferring slurry between reactors have been tried and the success of each of these options is examined. A conceptual design for an ideal bacterial oxidation reactor is given based on the assessment of current plant practice and available materials and equipment.