Modelling coal pyrolysis using conventional kinetic methods demonstrates that at temperatures below 1073K the conversion of coal into coke can be treated as two competitive processes which take place in two apparent areas of the kinetic and diffusion range. The relative arrangement of these areas reflects the coking ability of individual coals. It has been shown that the effect of the ash content upon the coking characteristics of the coals investigated can be evaluated using this kinetic-diffusion model.