Micrometer-sized (4-7 mu m diameter) poly(methacrylic acid) (PMAA) hydrogel microspheres were synthesized by precipitation polymerization. Individual microspheres were held in a micropipet and visualized by interference contrast microscopy. They Were characterized with regard to their mass, density, water content, electrophoretic mobility, and apparent pK(a). Equilibrium changes in volume were measured as functions of the pH and NaCl concentration of the suspending solution. The maximum reduction in the microsphere equilibrium volume (V-rmax) at pH 3.0 was 0.28, where V-r was the ratio of the microsphere volume at the test pH to its volume at pH 6.6. A Donnan-based thermodynamic model, modified to include counterion binding because of the high fixed charge density in the microspheres (3.0 M), was applied to determine the difference in the ion concentration between the interior and exterior of the gel. The ion concentration differences (which were related to the osmotic pressure) predicted by the model were proportional to the microsphere equilibrium volume with changing pH and salt concentration. This supported the hypothesis that the equilibrium volume of the microspheres was set by a force balance between the osmotic pressure and the elasticity of the hydrogel matrix. Microspheres changed from their maximum equilibrium volume at pH 6.6 to their minimum equilibrium volume at pH 3.0 in 300 ms. This indicated that diffusion of the polymer matrix and not diffusion of ions into and out of the microsphere was the rate-limiting factor in determining a microsphere＇s swelling rate.