In this study a new family of microgel particles is investigated which contain methylmethacrylate (MMA), ethylacrylate (EA), acrylic acid (AA), glycerol propoxytriacrylate (GPTA), and Ernulsogen (Em). GPTA is a trifunctional crosslinking monomer, whereas Em is a polymerisable alcohol ethoxylate surfactant. TEM and PCs data reveal that the extent of microgel swelling originates from a pH-independent contribution (due to Em) as well as a pH-dependent contribution (due to AA). The major contribution to swelling comes from pH-independent swelling. Consideration of the equations governing particle swelling allows the effective pK(a) of the incorporated AA groups to be estimated. There is evidence of a shift of the pKa for the AA groups from 4.5 to ca. 9.5 when the microgel particles containing AA also contain Em. This suggests intraparticle hydrogen bonding between AA and ethylene oxide segments at low pH. (C) 2003 Elsevier Science (USA). All rights reserved.