Phenanthrene, anthracene and pyrene have been pyrolysed in sealed reactors in the presence of coal. No methylation occurs at 220°C but increasing amounts of methylaromatics are formed at temperatures to 350 or 400°C. This methylation provides support for geosynthetic processes in sediments, because the experimental conditions used may be thermally equated to Ro values of 0.63–2.03%. The high concentration of n-alkanes generated from the coal may play a crucial role in such reactions. With increasing temperature there are systematic and theoretically predictable increases in the ratio of the β/α substituted methyl isomers of the parent aromatic hydrocarbons. The greater proportions of β-substituted isomers in the methyl products of reactions at higher temperatures may be due either to α→β rearrangements, or to direct methylation at the β-positions. Yields of methylphenanthrenes continue to increase up to 400°C. Lower yields of methylanthracenes and methylpyrenes at 400°C are attributed to demethylation.