Fibrous porous membranes composed of poly(N-isopropylacrylamide-co-acrylic acid) and poly(N-isopropylacrylamide-co-hydroxyethyl methacrylate) were prepared by electrospinning. The membranes behaved like hydrogels in water after being crosslinked at elevated temperatures. Investigations of the swelling and deswelling behavior indicated that the response rates significantly increased after the carboxyl and/or anhydride groups in the hydrogel membranes were ionized or neutralized with NaOH. It took them less than 60 s to reach equilibrium swelling and about 90 s to reach equilibrium deswelling. The response rates were much higher than those of the parent un-ionized membranes; this indicated that ionization played an important role in the ultrarapid response behavior. The response rate was also higher than that of most hydrogel materials previously reported and was even comparable to superporous hydrogels with high moisture contents. The mechanism of the ultrarapid response behavior of the ionized membranes was qualitatively analyzed. We believe that the fine fiber diameter, high porosity, and improved wettability with water contributed to the ultrarapid response behavior. This study presents a new and facile method for improving the response rate of hydrogel materials made by electrospinning. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115: 2485-2492, 2010