The injection of pressurized carbon dioxide (CO2) (gas or supercritical) in a liquid induces a lowering of viscosity the magnitude of which is a function of temperature and pressure. In this work, the addition of pressurized CO2 to highly viscous liquids is proposed as an ecologically viable and economically competitive technology to increase permeate flux during ultrafiltration at low temperature. Initially viscosity and phase-equilibrium data of model compounds, polyethylene glycols (PEG), in the presence of CO2 were studied at high pressure. Then tangential filtration runs with PEG-CO2 mixtures were conducted on pilot-plant units equipped with mineral membranes. Results thus obtained were analyzed using the previously measured kinetic and thermodynamic data.