Perfluoropolymer membranes have found commercial use because of their unique gas separation properties and chemical resistance. To date, studies of gas transport in perfluoropolymers have been limited largely to variants of the commercially available polymers, Teflon (R) AF, Cytop, and Hyflon (R) AD. Here, we describe gas transport in composite membranes fabricated from copolymers of perfluoro(2methylene-4,5-d imethy1-1,3-d ioxolane) and perfluoro(2-methylene-1,3-dioxolane). These new perfluoro copolymer membranes have superior gas separation performance compared to the commercial perfluoropolymers for a number of gas pairs, including H-2/CH4, He/CH4, N-2/CH4, and CO2/CH4. For example, membranes based on 57 mol% perfluoro(2-methylene-1,3-dioxolane) show H-2/CH4 selectivity of 130 combined with a H-2 permeance of 700 gpu. These values far exceed an upper bound for commercial perfluoropolymers. The transport properties of the new perfluoro membranes depend strongly on the copolymer composition with increasing amounts of the efficient packing monomer perfluoro(2-methylene-1,3-clioxolane), yielding membranes with higher size selectivity and lower per meance. (C) 2014 Elsevier B.V. All rights reserved.