poly(Acrylamide-co-N,N’-methylene bisacrylamide) microspheres were prepared and the sustained release of Fe3+ via ion exchange was investigated in response to the hydrochloric acid concentration, The experimental results revealed that the adsorption/desorption of Fe3+ with microspheres proceeds via ion exchange between Fe3+ and proton, The release rates of Fe3+ from microspheres were found to be moderate under proton concentrations below 10(-1) mol.dm(-3). However, an abrupt increase in the release rate was observed above 10(-1) mol.dm(-3). The permeability coefficient was proportional to the hydrochloric acid concentration and had a discontinuity at around 10(-1) mol.dm(-3). A on-off release mechanism from microspheres was demonstrated in response to a small change In the hydrochloric acid concentration between 10(-1) mol.dm(-3) and 10(-3) mol.dm(-3). The permeability coefficients were independent of the stirring speed, and the activation energy for sustained release was 63.9 Kj.mol(-1), indicating that sustained release was controlled by ion exchange of Fe3+ and proton.