The effects of aqueous-phase acidity and salinity on the isotonic swelling behavior of water-in-oil-in-water (W/O/W) emulsion liquid membranes (ELMs) under agitation conditions was studied. A non-ionic surfactant (EMSORB 2500) was used to stabilize the membranes. The acidity of the aqueous phase was controlled by sulfuric acid and the salinity was controlled by sodium sulfate. The swelling of the ELMs was monitored on a continuous basis using an on-line conductivity probe. The swelling rates of the ELMs were found to be strongly influenced by the aqueous-phase acidity and salinity. In order to elucidate the mechanism of swelling, the surfactant diffusivity was determined from dynamic interfacial tension measurements between the aqueous phase and the membrane phase (containing EMSORB 2500). The sedimentation rates of water droplets of a primary water-in-oil (W/O) were also measured to assess the stability of the water droplets. Based on the analysis of the results, it is concluded that the entrapment of external aqueous phase inside the membrane phase is due to droplet deformation and interfacial mixing mechanism. The net swelling rate of the ELM is equal to the difference between the entrapment rate and the escape rate of internal aqueous phase back to the external aqueous phase. The entrapment rate increases with the increase in surfactant diffusivity and the escape rate increases with the decrease in the stability of internal aqueous droplets. Both surfactant diffusivity and stability of internal aqueous droplets are strongly influenced by the acidity and salinity of the aqueous phase. (C) 2004 Elsevier B.V. All rights reserved.