In order to improve the separation efficiency of dispersed oil from water by hydrocyclones, a new process of utilizing air bubbles has been developed to enhance the separation efficiency. The air bubbles attach themselves to the oil droplets and cause a decrease in the overall density of the phase, the difficulty of agglomerating can thus be circumvented. The air-liquid ratio of 1% was found to provide the best separation. When the inlet Reynolds number ranges from 14,000 to 16,000, the oil removal efficiency increases from 72% (air-liquid ratio 0%) to 85% (air-liquid ratio 1%). The process has been found to be very efficient in the separation of suspended oil from water. Crown Copyright (C) 2010 Published by Elsevier B.V. on behalf of The Institution of Chemical Engineers. All rights reserved.