Hydrogen chloride and ammonia permeation resistance were measured through two grades of tetrafluoroethylene perfluoroalkoxy copolymers using standard manometric procedures. Hydrogen chloride showed a lower permeation coefficient than ammonia due to the combined effect of different molecular size and interaction with perfluoroalkoxy (PFA). The permeability coefficients of these two gases were comparable to those of oxygen and nitrogen. The two grades of PFA differed only in molecular weight or chain length. The molecular weight had little affect on permeation when the samples were cooled slowly. Slowly cooled samples always showed better permeation resistance than those cooled quickly. The influence of sample preparation was quite pronounced, demonstrating that process can be as important as polymer grade in determining the ultimate permeation resistance of Various PFA grades.