These are the findings from a computer model study of hydrogen gas leakage in buildings. The gas cloud shape and size was predicted using FLUENT 3.03. This study investigated the influence of building geometry and passive ventilation on the formation of combustible gas clouds. That is to say; at what leakage rates, and for what length of time, leaking hydrogen can be ventilated from a properly designed building without producing a sizable quantity of combustible fuel-air mixture. Copyright (C) 1996 Published by Elsevier Science Ltd on behalf of International Association for Hydrogen Energy