A commercial centrifugal rotary membrane module was used for the ultrafiltration of oil-water emulsions (droplet radius 50-3000 nm). This configuration can achieve high shear rates (>10(5) s(-1)) which are decoupled from the bull; recirculation rate. Fluxes were in the pressure controlled regime above 600 rpm with transmembrane pressures up to 345 kPa. The pressure dependent flux behaviour suggests that concentration polarization or gel formation was minimal. The dominant back transport mechanism was determined by comparing various back transport mechanisms to the permeation drag force. Back transport mechanisms included Brownian diffusion, shear induced diffusion, lateral migration, viscous drag, centrifugal and DLVO forces. The effect of the membrane surface porosity and Sherwood＇s correction for Stokes＇s law on the permeation drag were also studied. Viscous drag was the dominant force on droplet sizes between 50-1000 nm and was the only mechanism which could overcome the permeation drag force. Lateral migration was significant for droplets between 1000-3000 nm which were present in small quantities.