The state of California and the Environmental Protection Agency have developed AP-42 emission factors for prediction of atmospheric emissions from combustion of gasoline, diesel, fuel oil, and natural gas. This work develops a method to predict emission factors for the combustion of any substance, based on the kinetics of the combustion reaction. As part of the method to develop an emission factor for any individual substance, a dimensionless group is formulated which considers the ratio of the log-mean rate of combustion to the product of the space velocity through the combustion zone and the concentration differential between the precombustion and postcombustion states. This dimensionless group, which is referred to as Phi in this work, takes the value of approximately 530-780. The method provides emission factors which are in good agreement with those contained in the AP-42 documentation.