The current method for determining the flammable limits for a gas in a closed spherical vessel is based on a specification of the maximum pressure increase during the combustion, usually from 5 to 10% of the initial ambient pressure. This approach is completely arbitrary and is not fundamentally based. For most hydrocarbons this pressure boundary and hence The flammable limits are easy to determine experimentally since an abrupt pressure drop occurs at the flammable limits as the fuel concentration in air is adjusted. However, for some species, particularly hydrogen mixed with air, the drop in maximum combustion pressure is not very abrupt and the fuel concentration can range several percentage points depending on the arbitrary criterion used for the flammable limits. This article will discuss a new approach for determining the flammable limits for a gas in a spherical vessel. The approach is based on the maximum second derivative of pressure rise. The second derivative is indicative of an acceleration of the combustion process and is, hence, fundamentally based. Furthermore, we have identified a new approach to determine the downward propagating flammable limits based on the combustion time, that is, the time that the gas actually burns in the vessel. Experimental data for methane and hydrogen show that the second derivative flammable limit criterion produces slightly conservative values for combustion in air. Visual inspection of the combustion during the tests showed that no visual combustion was observed at the second derivative criterion. For methane, the second derivative criterion resulted in a flammable range in air front 4.6 to 15.8% methane. For hydrogen these limits are 3.6-75.2%. We believe that this method will provide a much more fundamentally based method to determine both the upper and lower flammable limits of upward flame propagation and also provide a means to determine the downward propagation limits in a spherical combustion vessel. (C) 2009 American Institute of Chemical Engineers Process Saf Prog 28: 227-236, 2009