The aim of this dissertation was to improve the biodegradation performance of a machine cutting oil employed in the processing of uranium at AWE. The machine cutting oil is chemically complex containing mineral oil, surfactants to improve emulsification and a phenolic biocide. Biodegradation was performed by a mixed culture of bacteria isolated from used cutting oil. Continuous, semi-batch and fed-batch systems were examined and stirred tanks and well-mixed biosupport reactors (immobilised cell systems) were compared. A continuous flow system was shown to provide a high rate of productivity with respect to COD removal. The extent of biodegradation would be important for a radioactive application and this ranged from 61-77% for hydraulic residence times of 10-53 hours, however it was noted that the extent of biodegradation was greater in the batch start-up period. The application of a Tween 80 emulsified feed to increase the bioavailability of the oil phase inhibited rather than improved the biodegradation performance. The use of a biosupport reactor improved biodegradation at shorter residence times. Development of these observations led to the operation of semi-batch (SBR) and fed-batch reactor systems (FBR). Following acclimatisation of the culture, the semi-batch systems consistently yielded higher extents of biodegradation in the range of 86-94% removal of COD for a bioprocess cycle of 5 days. The addition of n-hexadecane, as a readily metabolised co-substrate, to two semi-batch experiments did not noticeably increase biodegradation. A fed-batch configuration did not improve on the performance of the semi-batch configuration. GC-MS analysis of the extracted organics produced an unresolved complex mixture (UCM). Oxidation using chromium trioxide (CrO₃) of the UCM suggested that the remaining components were undegraded molecules from the base oil rather than excreted biodegradation intermediates. Phenotypic and genotypic assessment of the bacterial community showed that the overall diversity of the mixed culture was low. The bacterial population shifted throughout the batch period and diversity was further reduced as the cutting oil was degraded. Maximum diversity was observed at the start of the batch period and at steady state for the continuous system.