Abstract Samples of a coal and a lignite were boiled in pyridine, partially reduced and partially deuterated by modified Birch reductions. These samples were slowly pyrolysed in an inert atmosphere in a horizontal tube furnace to temperatures in the range 300–600 °C. The yields and the nature of the products are reported; in particular an attempt is made to characterize the various tars, and the pyrolyses are described in terms of a simplified mechanism. It was found that boiling the fuels in pyridine increased the subsequent yield of tar and increased the proportion of high-boiling material within the tar. Tar formation was diffusion-controlled. The role of hydrogen atoms from the hydroaromatic structure of the fuels in the formation of tar, liquor and gas was confirmed. Experiments with partially deuterated fuels established that during pyrolysis interchange of hydrogen atoms occurred amongst the molecules of the gas. Further pyrolyses of partially deuterated coals under wider conditions should be profitable.