This work studies the influence of temperature and hydrogen availability on the coprocessing of two high sulphur content Spanish lignites, with a petroleum vacuum residue. The experimental results show that temperature and partial pressure of hydrogen have a great influence on the conversion of coal and on the quality of oils formed. The conversion increases with temperature of reaction until a maximum is achieved. This maximum is between 400 and 420 °C depending on the reaction conditions. Beyond this maximum the conversion drops quickly, even producing negative results, due to recombination reactions. A higher hydrogen partial pressure induces a larger coal conversion but the maximum is achieved at lower temperatures. On the basis of these results, hydrogen transfer through aromatic compounds is proposed as the reaction mechanism of coprocessing.