The effect of perchlorate ions as counterions in the acid-induced swelling of poly(2-vinylpyridine) microgel particles was investigated. The pH was modified with perchloric, hydrochloric and hydrobromic acids and the particle diameter measured using dynamic light scattering and the viscosity monitored using a capillary viscometer. The microgel particles were found to have a lower apparent pK(a) with perchloric acid, 4.10 compared to 4.70 with hydrochloric and hydrobromic acids. As a result the particles swell at a lower pH and they also swell to a lower maximum diameter with perchloric acid, 681+/-8 nm compared to 751+/-15 nm for hydrochloric and hydrobromic acids. The swelling transition is also continuous with perchloric acid, whereas it has been identified as first order with hydrochloric acid. Similar variations were found in the viscosity using the different acids. The differences can be accounted for by the dehydrating and cross-linking nature of the perchlorate ion.