The purpose of the experimental work described in this paper is the determination of the ventilation requirements in enclosures containing fuel cells such that, in the case of a small non catastrophic release, the H(2) concentration in air for zone 2 ATEX (2% v/v) is not exceeded. A full scale fuel cell was placed inside the experimental facility having 25 m(3) volume. Three different leaks were investigated (40, 90 and 180 Nlt/min) and H(2) concentrations were measured at five locations inside the facility. Several vent areas were examined for the cases of natural ventilation. When natural ventilation failed to ensure H2 concentrations less than 2% v/v in the facility, mechanical ventilation using two fans was investigated. Based on the experimental set up, it was found that natural ventilation is sufficient when the air-flow calculated from ATEX guidelines is higher than 0.009 m(3)/s and the release flow rate corresponds to a non-catastrophic release, i.e. 40 Nlt/min. For higher release flow rates most of the ventilation configurations were not sufficient to maintain a H2 concentration less than 2% v/v. All forced ventilation configurations examined (together with the free ventilation areas used) were sufficient to maintain a H(2) concentration below 2% v/v for 40 Nlt/min and 90 Nlt/min release flow rates. For the higher release flow rate of 180 Nlt/min, most of the forced ventilation configurations were insufficient. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.