A linear mathematical model for forward combustion has been developed for underground gasification of coal. Predicted results agree very well with data obtained from a field test conducted by the Laramie Energy Research Center near Hanna. Wyoming. With the aid of the model, field tests can be interpreted quantitatively in terms of known chemical and physical phenomena. This leads to a greater understanding of the process, and establishes a method for determining optimum operating conditions such as air injection rate, air/water ratio and preheating of injection air.