A new air bag inflater based on the combustion reactions of methane-oxygen mixtures has been developed and modeled. The performance of this inflater was evaluated in terms of pressure-time relationships inside the inflater and in a receiving tank simulating an air bag as well as the temperature-time relationship in the tank. A theoretical model has been developed to simulate the transient pressure, temperature and mass flow rare from the inflater to the tank. The model is based on the change in the internal energy inside the inflator and the receiving tank as the mass flows from the inflater to the tank. The model utilizes the Chemical Equilibrium Compositions and Applications code developed by NASA to estimate the equilibrium conditions in the inflater and the mass fractions of the product species. The model predicts the pressures and temperatures inside the inflater and the tank as a function of time. The predicted results were in good agreement with the experimental results. This model can predict transient pressures and temperatures with an accuracy of +/-15%. The present modeling approach can be applied to a range of combustible gas mixtures, including hydrocarbon-oxygen, hydrogen-air and others.