The reactions between solids the rates of which are greatly enhanced by gaseous intermediates are described in this article. In recent years many important industrial processes have been developed and may be classified as one of the reactions of this type. Notable examples include the "direct" reduction of iron ores by carbon or coal as an alternative to the conventional blast furnace process. The formations of carbides are further examples. Recent developments on the mathematical analysis of this type of a system are discussed. In Part IV the treatment of multiparticle systems encountered in industrially important processes will be presented.