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Catalytic activity of sulfated zirconia was affected by both sulfur loading level and surface crystal phase. Incorporating silica or alumina into the catalyst system stabilized the tetragonal crystal phase and prolonged activity. Stabilized sulfated zirconia prepared with 20 wt.% alumina maintained catalytic activity for the isoparaffin alkylation reaction six times longer than pure sulfated zirconia. This performance enhancement has, thus far, not been disclosed in the literature. Reasons for the enhanced performance are hypothesized. UV–Raman spectroscopy was used to characterize the surface structure of sulfated zirconia. Samples displaying tetragonal reflections were catalytically active, while those displaying a surface monoclinic phase showed little activity. Examination of fresh, spent, and regenerated catalyst did not reveal any evidence for a phase transformation, indicating catalyst deactivation was most likely caused by the accumulation of hydrocarbon fragments on the surface.