Thermosensitive microgel dispersions of poly(N-isopropylacrylamide), having either cationic or anionic surface charge groups, are shown to absorb significant amounts of ions from aqueous solution at 25 °C. Under these conditions the microgel particles can be envisaged as sponge-like materials having a spherical conformation consisting of numerous interstitial spaces. At 50 °C, a reversible contraction of the microgel takes place, resulting in a reduction in volume thereby causing a large amount of the retained material to be released from within the closed interstitial spaces of the structure. The effective charge density on the particle surface simultaneously increases under these conditions. The nature of the ionic initiator used in the polymerization of the microgel particles has a considerable influence on the affinity of each ionic species, hence influencing the total amount of material absorbed and also the kinetics of the absorption process.