Abstract: A process for improving the gas separation efficiency of a bundle or module of hollow gas separation fibers by selectively sealing defective fibers without sealing good fibers through which a gas A passes significantly more readily than a gas B. At the beginning of this process the same atmosphere containing A and B is in the interior and at exterior surfaces of said fibers. A curable sealant is applied to a first open end of the fibers, the opposite end of the fibers is blocked, and the partial pressure of A at the exterior fiber surfaces is increased. The resulting pressure build-up inside the non-defective fibers prevents them from being penetrated significantly by the sealant. When the tip of the fibers is cut cleanly away, the good fibers are open but the defective fibers are plugged with sealant.