pH sensitive hydrogels with semi-interpenetrating networks (semi-IPN) composed of cross-linked copolymer of acrylamide/acrylic acid (P(AM-co-AA)) and linear polyallylammonium chloride(PAAC) were synthesized by template copolymerization in the presence of N,N'-methylene bisacrylamide (MBAM) as a cross-linking agent. The networks contained both covalent bonds and ionic bonds. The covalent bonds retained the three-dimensional structure of hydrogel and the ionic bonds brought the hydrogel with much higher mechanical strength and pH stimuli responsive reversibility. The effect of the molar fraction of PAAC (based on monomeric units) on the swelling-deswelling transition pH and reversibility behavior of hydrogel was investigated by equilibrium swelling and oscillatory swelling techniques. The results showed that the semi-IPN hydrogels exhibited ampholytic and reversible pH-stimuli responsive characteristics. The swelfing/deswelling transition pH value could be varied in a great rang by modulating the PAAC contents at the molar fraction of AA (f(AA)) fixed or varied. The drug loading and release of the semi-IPN hydrogels were also studied using theophylline as a model drug. The perfect reversibility and rapidly on-off switching properties of the hydrogels make them to be potential materials in the field of medicine, pharmacy and biotechnology. (c) 2005 Elsevier Ltd. All rights reserved.