Alumina-supported iridium catalysts were prepared with highly dispersed iridium particles. The surface acidity of the catalysts was varied by addition of variable amounts of HCl to the impregnating solution and also by using an organometallic precursor. The catalysts were used in toluene hydrogenation and in methylcyclohexane dehydrogenation reactions and the thiotolerance level was measured with a feed containing 10 ppm of thiophene. In both reactions, it was found that the sulphur tolerance increases with increasing acidity of the support.