Regarding UF retentions, the tannin rejections (74% and83% for OT050 and GR60PP membranes at 5 bar, respectively)were much higher than expected considering the molecular weight distribution of vegetable tannins (500–20,000 Da) and the MWCO of the membranes (Table 2). The high rejection observed for both membranes could be based on the size exclusion mechanism separation and could be attributed to particular properties of vegetable tannins extracts. They are well miscible with water to form polydisperse solutions, partially of colloidal type. In aqueous solution, labile aggregates are formed from non-associated molecules