Membrane fouling has been identified as one of the major problems during the treatment by ultrafiltration (UF) of the cork processing wastewaters. This problem leads to the drastic reduction of the UF permeate fluxes and is often attributed to the fact that these wastewaters are rich in phenolic/tannic colloidal matter. In order to understand and minimize membrane fouling, this work addresses a systematic evaluation of the ultrafiltration of cork processing wastewaters, in terms of UF membrane characteristics, UF operating conditions and consideration of an integrated process of ozonization/ultrafiltration. The goal for the ozonization pre-treatment is to reduce the phenolic/tannic colloidal matter content and correlate that with the membrane fouling and the UF permeate fluxes enhancement. Five ultrafiltration membranes were investigated: a commercial membrane, TS60 (Ropur) and four laboratorymade cellulose acetate membranes. They cover a wide range of molecular weight cut-offs, from 4 to 98 kDa. The ultrafiltration experiments are carried out in flat-sheet cells of 13.2 cm2 of membrane surface. The UF operating conditions are varied in terms of the hydrodynamics, circulating velocities of 100 l/h and 200 l/h, and in terms of transmembrane pressures, from 1 to 3 bar. The ozone pre-treatment led to a reduction of 11% in the total organic carbon content and to a reduction of 51% in the total polyphenols content, these reductions led to the enhancement of the UF permeate fluxes.