Carbon monoxide adsorption has been investigated on Pt particles supported on a high surface area zirconia and sulfated zirconias. The accessibility of the Pt surface determined from the comparison of H-2 chemisorption and transmission electron microscopy depends on two parameters : the temperature of treatment in air used to dehydroxylate sulfated zirconia, and the temperature of reduction. An oxidative pretreatment at 823 K yields a poor accessibility of Pt (0.03 < H/Pt < 0.05) whatever the temperature of reduction, whereas a Pt dispersion of 0.6 can be obtained by oxidation at 673 K followed by a mild reduction at 473 K. FTIR spectroscopy of adsorbed CO on Pt/ZrO2 shows besides the normal linear species at 2065 cm(-1), a band at 1650 cm(-1) which is attributed to CO bridged between Pt and Zr atoms. On Pt/ZrO2-SO42-, all bridged species tend to disappear, as well as the dipole-dipole coupling and nu(CO) is shifted by 57 cm(-1) to higher frequencies. These results are attributed to sulfur adsorption on Pt which decreases the electron back-donation from Pt to the 2 pi* antibonding orbital of CO. The lower initial heat of CO adsorption observed on Pt/ZrO2-SO42- supports this proposal.