This work deals with cross-flow ultrafiltration of highly viscous liquids. Several years ago a few attempts were made to decrease viscosity by either using high temperatures or adding chemicals. But these alternatives present significant drawbacks: high additional costs in terms of equipment or energetic consumption and destruction of thermosensitive compounds for the first, or additional separation to preserve environment for the second one. The new idea that we have developed is to reduce liquid viscosity at ordinary temperature by injecting a gas (CO2) in supercritical conditions before ultrafiltration. At the end of the membrane process, the gas may be freely eliminated without environmental damage by a simple pressure reduction, which constitutes a main advantage. Based on this idea, very promising experiments were recently conducted at temperature conditions ranging from 40 to 80degreesC. The mixture viscosity is reduced and ultrafiltration performed with suitable inorganic membranes gives significant transmembrane fluxes. The process is tested on model oil. We recently observed membrane fouling when working below the critical pressure of CO2. Mechanism has been proposed to give an account of these results.