The isomerization reaction of n-pentane over ion-exchanged and platinum loaded zeolite (Molecular Sieve 13x) catalysts was studied under hydrogen pressure. Zeolite catalyst ion-exchanged with Ca, Zn, Mg, or Mn cation had poor catalytic activity. On the other hand, rare-earth cation-exchanged catalysts had excellent catalytic activity in the case that the catalysts were reduced and dried under hydrogen gas flow at low temperatures. But the activity of the catalysts was decreased by high temperature reduction and drying with hydrogen gas flow. The decreased catalytic activity was restored by isopropyl alcohol addition to feed n-pentane, or by steam treatment of the catalysts. These results suggest that active sites of the catalysts are formed by chemisorption of water, and water is reversibly chemisorbed on and desorbed from the catalyst surface.