Microbubble dispersion stability is a desirable characteristic in applications such as separation processes and in-situ bioremediation. This study investigates the effects of surfactant concentration, pH and ionic strength on the stability of dispersions of rhamnolipid, a common anionic biosurfactant. Microbubble dispersions of rhamnolipid and the non-ionic synthetic surfactant tergitol 15-S-12 were prepared by intensive stirring at 8000 rpm with solutions of 500-4000 mg 1(-1) surfactant concentration at pH 6-8. The ionic strength tests were performed with 1000-3000 mg 1(-1) sodium chloride. Dispersion stability increases for higher surfactant concentrations, but decreases with rising pH and increasing salt concentration. However, increasing the pH in the co-presence of salt enhances dispersion stability. A modified model showing improved fits to liquid drainage from the dispersions is presented and it is shown that liquid drainage occurs in three distinct phases, instead of two phases as previously assumed in the literature. (C) 2009 Elsevier Inc. All rights reserved.