Swelling equilibrium and kinetics of swelling for hyaluronic acid (HA) benzyl ester membranes were examined in media of different pH and ionic strength, and for varying percent esterification of the polymers. The equilibrium swelling properties for polymers with an intermediate (75-85%) degree of esterification in different media can be explained by the Donnan equilibrium theory. However, this theory does not apply to the swelling behaviors of membranes with high (> 85%) and low (< 75%) degrees of esterification, implying that non-ideal conditions must also be considered. The kinetics of swelling in media of different ionic strength can be described by a second-order swelling model, with both diffusion and stress relaxation contributing to the process. Ion exchange influences the swelling kinetics of HA ester membranes at lower pH in the medium.