Efficient water electrolyzers that generate high-pressure hydrogen and oxygen are useful for aerospace and civilian applications. A lightweight polymer-electrolyte-membrane electrolyzer capable of producing oxygen and hydrogen at a balanced pressure of 13.8 MPa has been designed and fabricated by Hamilton Sundstrand Space Systems International (HSSSI). This electrolyzer uses a unique metal-laminate cell design that reduces the weight of traditional high-pressure water electrolysis stacks by eliminating the need to place the stack within a pressure vessel. The electrolyzer operates on water fed to the anode and cathode (dual-feed mode). This design simplifies the balance of plant for a regenerative fuel cell system and enhances system safety. The electrolyzer was tested at balanced pressure in the range of 1.72-13.8 MPa using a test system designed and constructed at the Jet Propulsion Laboratory. Current efficiency of 96 to 97% was achieved at a balanced product pressure of 13.8 MPa. The polarization characteristics were governed by the kinetics of oxygen evolution and the ionic resistance of the membrane electrolyte. The electro-osmotic water drag coefficient was determined to be in the range of 2.3-2.4 and this value did not vary with the operating pressure up to 13.8 MPa. The dual-feed electrolyzer is a promising design for achieving high energy efficiencies at high product pressure. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.038111jes] All rights reserved.