Abstract Thermal production of hydroaromatics by addition of molecular hydrogen to polyaromatics was studied in the presence of hydrogen sulphide or tetralin. The results indicate the importance of wall catalytic effects related to H 2 H 2 S activity during fossil fuel treatment. Yields of partly hydrogenated polyaromatic structures, potential H-donor sources for fossil fuel, treatment were improved by H 2 H 2 S pressure under batch reactor conditions, with selectivity maintained in production of dihydro aromatic forms. The reduction mechanism is shown to be related to wall effect with probable formation of H ∗ and SH ∗ reactive species. This effect is an important parameter for the use of stainless-steel reactors.