An integrated system combining highly efficient III-V solar cells in an optical concentrator system with a polymer electrolyte membrane electrolyser is presented. Hydrogen and oxygen are produced by solar water splitting. The integrated design and the high concentration factor of 500 allow material savings and high system efficiencies. The thermoneutral solar to hydrogen conversion efficiency of a prototype system with an area of 96 cm(2) was measured outdoor to be 18%. (C) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.