The prediction of cavity growth is important for selection of borehole spacing in underground coal gasification. A one-dimensional model based on chemical reaction, mass transfer, and heat transfer has been developed to predict the lateral advance of a reaction front through a coal face. Two of the important zones in a burning coal face, namely the drying and pyrolysis zones, have been analyzed in some detail, and the effects of important variables in these zones have been determined through computer studies. This model can also be expanded to three dimensions to simulate the evolution of the burned out space in a coal seam of uniform thickness.