In this paper, safety considerations related to the operation of proton-exchange membrane (PEM) water electrolysers (hydrogen production capacity up to 1 Nm(3)/h and operating pressure up to 130 bars) are presented. These results were obtained in the course of the GenHyPEM project, a research program on high-pressure PEM water electrolysis supported by the European Commission. Experiments were made using a high-pressure electrolysis stack designed for operation in the 0-130 bars pressure range at temperatures up to 90 C. Besides hazards related to the pressure itself, hydrogen concentration in the oxygen gas production and vice-versa (resulting from membrane crossover permeation effects) have been identified as the most significant risks. Results show that the oxygen concentration in hydrogen at 130 bars can be as high as 2.66 vol %. This is a value still outside the flammability limit for hydrogen oxygen mixtures (3.9-95.8 vol %), but safety measures are required to prevent explosion hazards. A simple model based on the diffusion of dissolved gases is proposed to account for gas cross-permeation effects. To reduce contamination levels, different solutions are proposed. First, thicker membranes can be used. Second, modified or composite membranes with lower gas permeabilities can be used. Third, as reported earlier, external catalytic gas recombiners can be used to promote H-2/O-2 recombination and reduce contamination levels in the gas production. Finally, other considerations related to cell and stack design are also discussed to further reduce operation risks. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.