The effect of sulphur on the coking of Rh/Al 2O 3 in the steam reforming of 1-methylnaphthalene was studied at 440–500°C, 1 atm and a steam-to-l-methylnaphthalene molar ratio of 14. Three grades of 1-methylnaphthalene were used, containing 690, 80 and < 5 ppm of sulphur. In all instances the presence of sulphur compounds considerably increased the coking rate. The profiles of coke and of sulphur in the catalyst bed and inside the pellets were very similar, which explains why powder catalysts were deactivated much faster than pellet catalysts. The role of sulphur has not been entirely elucidated and could be due either to rapid cracking of the sulphur compounds themselves or to inhibition by sulphur of the carbon-steam reactions. These two hypotheses are discussed in the light of other studies concerning the mutual influence of coke and sulphur in the deactivation of metal catalysts.