Ultrafiltration is widely used in the chemical, pharmaceutical, food and water industries. Practical difficulties arise in designing and operating the process due to concentration polarisation and membrane fouling. Enhancement of ultrafiltration is highly desirable to achieve a higher permeate flux at a fixed energy input, or a reduced energy input whilst maintaining the level of permeate flux, or an improved selectivity of the membrane. One effective, simple, and economic technique used to enhance ultrafiltration is the use of gas bubbles, ie injecting gas into the feed stream to create a gas-liquid two-phase cross-flow operation. In this paper, an attempt is made to compare the effect of 'bubbling' on the ultrafiltration performance, using different membrane modules (in particular, tubular and hollow fibre membrane modules). The difference in performance can be related to the feature of two-phase flow hydrodynamics and its respective effect on mass transfer. The advantages and drawbacks of using this technique to enhance ultrafiltration are discussed. (C) 2003 Society of Chemical Industry.