To estimate the hydrogen mobility of solvents quantitatively, reactions of tetralin, decalin and naphthalene with tritiated gaseous hydrogen in the presence and absence of H2S were performed under the conditions generally used for coal liquefaction (400 degrees C, initial hydrogen pressure 60 kg/cm(2)). The conversions of tetralin and decalin to other organic structures in the presence and absence of H2S were very close to each other. However, the conversion of naphthalene in the presence of H2S was promoted about four times of that in the absence of H2S. In the absence of H2S, the hydrogen exchange ratio (HER: the ratio of hydrogen exchanged in a solvent to total hydrogen in the solvent) of naphthalene was highest among the solvents. However, in the presence of H2S, the HERs of tetralin and decalin were promoted about 10 times. In the presence of H2S, the HER of tretalin was highest in comparison with those of decalin and naphthalene. It was suggested that H2S promoted the hydrogen exchange through tetralyl radical.