In order to elucidate the influence of the crystal structure of zirconia on the properties of the metallic and acid function of Pt/SO42- -ZrO2, catalysts with different zirconia crystal phases were synthesized, fully tetragonal, fully monoclinic, and with a mixture of the tetragonal and monoclinic phases. Their catalytic properties were studied in the metal-catalyzed reaction of cyclohexane dehydrogenation (300 degreesC, 0.1 MPa, WHSV = 10 h(-1), H-2/C6H12 = 30), the acid-catalyzed isomerization of n-butane (350 degreesC, 0.1 MPa, WHSV = 1 h(-1), H-2/C4H10 = 6), and the bifunctional hydroconversion of n-octane (300degreesC, 1.5 MPa, WHSV = 4 h(-1), H-2/C8H18 = 6). TPR, XRD and FTIR of chemisorbed CO were also used in order to characterize the catalysts. The results showed a strong influence of the crystal phase on the activity of the acid function. A less marked effect was found for the metal-catalyzed reaction. An opposite relation between the two functions was seen with respect to this crystal structure influence. Among the sulfated catalysts, monoclinic Pt/SO42- -ZrO2 had the lowest activity in n-C-4 isomerization and the highest activity in cyclohexane dehydrogenation. Tetragonal Pt/SO42--ZrO2 catalysts were the most active in isomerization of n-butane. They had the lowest activity in cyclohexane dehydrogenation and their metal properties were negligible. They were also the most active in n-C-8 conversion, producing mainly i-C-4. Monoclinic catalysts had low cracking activity and produced mainly isooctane. Mixed phase catalysts had an intermediate behavior. While S poisoning of Pt was present as a uniform effect on all sulfated catalysts, the metal-acid behavior and the different Pt properties could be explained by a metal-support interaction between tetragonal SO42--ZrO2 and Pt. The Pt-support interaction was analyzed both with a model of Pt particles encapsulation and a model of electron depletion. The electronic deficiency of Pt particles supported on SO42--ZrO2 was evident in the shift of the IR bands of adsorbed CO. Further experimental work is however needed to get conclusive evidence about the nature of the Pt-support interaction. (C) 2004 Elsevier B.V. All rights reserved.