Isomerization of n-hexane (1-5 bar) is studied over sulfated zirconia at 423 K and 50 bar, under 5-45 bar hydrogen. The sulfated zirconia is crystallized at 923 K, then loaded with 0.025-0.8 wt% platinum. The isomerization rate first increases with hydrogen pressure, reaches a maximum, and finally decreases slowly. At a given platinum content, the position of the maximum increases with hexane pressure. The shape of the activity curves versus hydrogen partial pressure depends strongly on the platinum content. All the activity data are kinetically modeled by an acid mechanism in which the Lewis acid-base sites create carbenium ions. Activated hydrogen on platinum spills over to form the hydride species responsible for the desorption of the isocarbenium ions. The three parameters of the rate equation reflect the number of sites, the acid strength, and the efficiency of platinum. In the series of catalysts examined, the acidity is constant while the platinum efficiency increases with loading and reaches an upper limit at 0.15% platinum loading. (C) zool Academic Press.