The effect of sulfonation and bromination-sulfonation on the gas transport properties of polyphenylene oxide has been investigated. These high-performance modified polymers have been studied in the form of TFC membranes by solution coating on the skin side of polyetherimide hollow fibers. TFC membrane modules prepared from sulfonated-brominated polyphenylene oxide as the active layer coated on polyetherimide hollow fibers. Stability of the TFC membranes was greatly improved when a wet feed stream was used instead of a dry one. Water vapor in the feed stream most likely prevented the active layer from stress cracking on drying.