Poly(phenylene oxide) (PPO) membranes with sulfortic groups were prepared by two methods: plasma-initiated surface grafting of sodium styrenesulfonate or chemical sulfonation (with chlorosulfonic acid) of PPO, followed by membrane preparation from the modified material. The presence of ionic groups greatly increased the surface polarity of the material and the flux of water through the membranes. It seemed, however, not to improve the protein filtration process. Coating of these membranes with allylamine plasma polymer resulted in amphoteric membranes much better for ultrafiltration than those made of unmodified PPO. Such bipolar membranes appeared to be also very effective in micellar-enhanced ultrafiltration of mixtures of the 2,4-D herbicide and hexadecyltrimethylammonium bromide.