Large quantities of flammable gases are used in both commercial and residential applications, The risk associated with the use of these materials depends on an understanding of the impacts of an explosion, particularly the pressure-time history during The explosion, This work provides a model to calculate the pressure-time history fir an explosion in a closed, spherical vessel. The model is expected to be of great. benefit for characterizing these explosions and also for The design of explosion vents and other explosion Protection systems. The unique features of this model are that it takes into account The effect of two beat capacity ratios, unburned gas and burned gas, and assumes a variable burning speed-both, features are verified by experimental data to be important. The model requires the following parameters: chemical equilibrium, constant volume explosion pressure.. beat capacity ratio for the gas as a junction of temperature; and an exponent for the flame speed as a function of pressure and temperature, Application of the model to experimental data results in a single adjustable parameter called the "burning Parameter." The model was verified for both hydrogen and methane combustion. The results show that the model can accurately predict the experimental pressure-time curves over almost the entire pressure time history from initial ignition tip to the maximum pressure rise. This is a substantial improvement over previous models, which could only represent the data over a small time period or small pressure increase. (C) 2009 American Institute of Chemical Engineers Process Saf Prog 28: 141-146, 2009