With an ever-increasing population and rapid growth of industrialization, there is a great demand for fresh water, especially for drinking. In view of this, different desalination technologies have evolved with a thrust for utilization of renewable energy sources such as solar energy, ocean thermal energy, geothermal energy, and waste heat. Flash desalination is a technology in which fresh water is produced from seawater by evaporation and subsequent condensation. One such desalination technique involves different processes like pressurization of seawater using a pump, creation and maintenance of vacuum using jet pump in a flash chamber, evaporation of seawater at reduced pressure using warm water from the sea surface and condensation of water vapour using cold water from the deep sea. A two-phase variable geometry jet pump was designed, developed and tested for the flash desalination system to create a vacuum inside the flash chamber. The area ratio of the jet pump used for the investigation was 0.25. Experimental results are reported from tests on the jet pump with a view to assessing the optimum distance between orifice exit and mixing tube entrance. Overall performance of the jet pump was assessed by plotting its characteristics. Performance of the jet pump was analyzed by plotting iso-efficiency curves on characteristics curves drawn for various primary water flow rates. The optimum orifice to mixing tube distance of the jet pump was found and corresponding maximum efficiency was reported. Maximum vacuum created by the jet pump at an optimum condition, i.e., at maximum jet pump efficiency, with respect to time was determined. © 2006 Elsevier B.V. All rights reserved.