Microcalorimetry and infrared spectroscopy were used in conjunction with reaction kinetics measurements to investigate the effects of catalyst hydration on the activity for n-butane isomerization at 423 K of sulfated zirconia. The catalytic activity for a sample dried at 773 K is an order of magnitude lower than that of a sample dried at 588 K. However, the catalytic activity for the sample dried at 773 K is promoted by dosing quantities of water equal to approximately 75 mu mol/g onto the surface at 423 K. Larger doses of water poison the catalyst. The acidity of the catalyst after drying at either 588 or 773 K was almost entirely of the Bronsted acid type. Dehydration of the catalyst did not alter the heat or the extent of ammonia adsorption. Water dissociates upon adsorption on the sample dried at 773 K, producing new hydroxyl groups whose presence is critical for n-butane isomerization on sulfated zirconia.