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The use of hydrocvclones for deoiling produced water has grown steadily over the past ten years. Although hydrocvclones have been used as liquidliquid separators for more than forty years, the need to maintain and hrther reduce the discharge concentration of dispersed oil in produced water below 20-30ppm has motivated fbrther research which aims to identi6 improved designs and optimal operating conditions for deoihg hydrocyclones. Hydrocyclones with either tangential or involute feed entries are commonly employed as deoilers? but they are limited by drop breakup at high entry velocities. This phenomenon has been hrther quantlfied in laboratory tests conducted at room temperature on a commercial deoiling hydrocyclone for feed rates between 50 and 150 liters per minute. Kerosene in water was used as the experimental dispersion with the mean particle size in the feed stream ranging from 15 to 60 microns. The cut-size of the hydrocyclone studied was determined by measuring the particle size distributions of the feed and underflow streams using a laser light scattenng techaue. Due to drop breakup within the hydrocyclone, the 50 % cut-size increases sigdicantly for feed dispersions with mean drop sizes larger than 20 to 35 microns.