Highly active sulfated zirconia (SZ) materials were prepared by sulfation of crystalline zirconia with gaseous SO 3. This direct method circumvents the use of liquid sulfation agents, as well as a final calcination step. The amount of sulfur species retained by SO 3-SZ samples was directly proportional to the fraction of monoclinic phase and the concentration of Brønsted acid sites. However, the catalytic activity in n-butane isomerization at 373 K was proportional to the concentration of labile chemisorbed SO 3, most likely in the form of pyrosulfate species showing a characteristic IR band at 1404 cm −1. Tetragonal zirconia favors the formation of these labile sulfur species, playing a key role in light alkane activation.