This paper describes our work to predict the flammability zone for any mixture of fuel, oxygen and nitrogen. The method utilizes a commercially available equilibrium program to determine the calculated adiabatic flame temperature (CAFT). The model is compared with our extensive experimental data obtained in a 20 L sphere at an initial pressure of 1 atm and 298 K. The data and model compare well over the entire flammability zone for two pure species, methane and ethylene, and not as well for a 50/50 mixture of methane/ethylene. Our results show that a good prediction of the flammability zone is obtained using a CAFT criterion of 1200 K: Furthermore, the intermediate species and solid carbon must be included in the equilibrium calculation to fit the fuel rich part of the zone. The intermediate species were selected by the equilibrium program using a built-in species selection criteria. For a gas mixture of methane and ethylene, we were unable to identify mixing rules for estimating K-G and P-max from pure component data. This method provides a direct approach to full flammability zone prediction.