The catalytic activity in the isomerization of butane of a variety of zirconia sulfate catalysts, both unpromoted (AZS samples) and promoted with Pt (PZS samples), prepared by an aerogel technique, is reported. Catalysts have either different sulfate contents (AZS samples) or different Pt content and constant sulfate content (PZS samples). Catalysts are studied in a continuous flow system at 150 and 250 degrees C, both in the absence and in the presence of hydrogen in the feed. AZS samples have identical specific activity at 150 degrees C, while differences are observed at 250 degrees C both in the absence and in the presence of H-2. PZS samples are more active and stable, particularly when activated in air only. No effect of Pt content on the catalytic activity is observed in the range explored (0.65-4.96%), while a negative effect of hydrogen can be evidenced under suitable conditions. An FTIR analysis performed on both AZS and PZS samples using a procedure that mimick the catalysis conditions suggest that contamination of the active sites on stream is mainly due to water and, in the absence of H-2 in the feed, to coke or oligomers deposition. A reaction mechanism is proposed involving an intermediate intermolecular butane dimerization where Zr4+ Lewis acid centers promote the initial C4 carbenium ion formation, followed by addition to in situ formed butene. The roles of Fit in the catalysis are also discussed.