Polymer electrolyte membrane (PEM) water electrolysis has demonstrated its potentialities in terms of cell efficiency (energy consumption ca. 4.0-4.2 kW/Nm(3) H(2)) and gas purity (>99.99% H(2)). Current research activities are aimed at increasing operating pressure up to several hundred bars for direct storage of hydrogen in pressurized vessels. Compared to atmospheric pressure electrolysis, high-pressure operation yields additional problems, especially with regard to safety considerations. In particular the rate of gases (H(2) and O(2)) cross-permeation across the membrane and their water solubility both increase with pressure. As a result, gas purity is affected in both anodic and cathodic circuits, and this can lead to the formation of explosive gas mixtures. To prevent such risks, two different solutions, reported in this communication, have been investigated: first, the chemical modification of the solid polymer electrolyte, in order to reduce cross-permeation phenomena; second, the use of catalytic H(2)/O(2) recombiners to maintain H(2) levels in O(2) and O(2) levels in H(2) at values compatible with safety requirements. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.