The behaviour of hydrogen in naphthalene in the pyrolysis of coal tar was investigated using a tritium tracer technique to elucidate the pyrolysis mechanism of coal tar. The pyrolysis of coal tar containing tritiated naphthalene was carried out using a batch reactor at 800-950 degrees C for 50 sec. The ratios of amounts of naphthalene in the tars pyrolysed at 800, 900, and 950 degrees C to that in the feed tar were 80.1, 73.4, and 42.5 wt%, respectively. However, the ratios of the radioactivity of naphthalene in tars after pyrolysis to that in the feed tar also decreased significantly with increasing temperature and it was only 12.9% for the reaction at 950 degrees C. The ratio of the recovered radioactivity of naphthalene to the initial radioactivity of naphthalene in the feed tar was about one-third of that of the recovered amount of naphthalene to the initial amount of naphthalene in the feed tar at 950 degrees C. The results indicated that the hydrogen in naphthalene transferred to other compounds in the pyrolysis of coal tar. It is suggested that naphthalene could play an important part in the radical reaction mechanism in the pyrolysis of coal tar.