in this work, a study on a perfluoropolymer purification with crossflow MF and UF, using commercial membranes, has been carried out. Copolymers of tetrafluoroethylene (TFE) and 2,2,4trifluoro, 5trifluorometoxy 1,3 dioxide (TTD), known commercially as HYFLON(R) AD, are highly transparent to light from deep UV to near infrared, so they find applications in optic and electronic industries, such as plastic optical fibers (POF), anti-reflective coating and protective pellicles in manufacturing semi-conductor. For the above application, it is often crucial to avoid the presence of both suspended and dissolved contaminants in the polymer and polymeric solutions. Membranes made from this amorphous perfluoropolymer were prepared in flat sheet, tubular and hollow fiber forms. Tests of membrane hydrophobic character and of pure gas permeability were carried out. Experimental gas separation data obtained with membranes prepared with TTD-TFE co-polymers and data from the literature on membranes made with co-polymers of perfluoro2.2dimethyldioxide (PDD) and TFE, commercially known as TEFLON(R) AE revealed an interesting linear relationship between permeation and glass transition temperature T-g. The voids volume fraction (Phi(v)) of the above amorphous perfluoropolymers was also estimated from the difference between the experimental polymer density and a theoretical density obtained by simple calculations using the group contribution method,