Studies on the effect of surfaces on the extent of the flammable cloud of high-pressure horizontal and vertical jets of hydrogen and methane are performed using CFD numerical simulations. For the horizontal jets, two scenarios pertaining to the location of the surface are studied: horizontal surface (the ground), and vertical surface (side wall). For a constant flow rate release, the extent of the flammable cloud is determined as a function of time. Effects of the proximity of the surface on the flammable extent along the axis of the jet and its impact on the maximum extent of the flammable cloud is explored and compared for both hydrogen and methane. The results are also compared to the predictions of the Birch correlations for flammable extents. It is found that the presence of a surface and its proximity to the jet centerline result in a pronounced increase in the extent of the flammable cloud compared to a free jet. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.