Abstract The results of a large test series on vented gas explosions are herein reported (with vessel sizes ranging from 1 to 100 m 3 mixture of methane, propane, and hydrogen with air). The pressure was not only measured internally but also externally. The flame propagation was also recorded with a speed camera. It is shown that a very significant explosion could result from the formation and ignition of the combustible cloud outside the vent and that, in a number of circumstances, this “external” explosion would overcome the internal one. Surprising results were also obtained showing, for instance, that near the vent the maximum overpressure may be lower than further downstream. This is not because the external explosion is developing far downstream but because of the local influence of the chamber pumping the outside atmosphere after the flame went out (thus creating a lower than atmosphere pressure) produced at the vent. The mode of combustion of the external cloud is still outstanding and cannot be described by the traditional outward expanding flame kernel. © 2009 American Institute of Chemical Engineers Process Saf Prog, 2010