Coal is a heterogeneous organic substance which is not meltable without decomposition and almost totally insoluble in common organic solvents. It does not readily react chemically under mild conditions and therefore is very resistant to microbiological degradation. In deciding whether coal can be converted into useful low-molecular weight products by microbial action, the strength of the chemical bonds in the coal structure must be considered. To make this determination, the results of coal solubilisation by different methods of increasing severity and the properties of the products obtained are discussed. The conclusion is that the coal structure consists of a macromolecular three-dimensional network with strong carbon-carbon bonds penetrated by a principally soluble but still high-molecular ＇mobile phase＇. This system is accessible to microbial attack only to a very small extent. However, if the microorganisms are fed with soluble products from chemical degradation or derivatisation of the coal substance, microbial attack may be possible.