Infrared spectra of adsorbed CO have been used as a probe to monitor changes in Pt site character induced by the coking of Pt/Al2O3 and Pt-Sn/Al2O3 catalysts by heat treatment in heptane/hydrogen. Four distinguishable types of Pt site for the linear adsorption of CO on Pt/Al2O3 were poisoned to different extents showing the heterogeneity of the exposed Pt atoms. The lowest coordination Pt atoms (nu(CO) < 2030 cm(-1)) were unpoisoned whereas the highest coordination sites in large ensembles of Pt atoms (2080 cm(-1)) were highly poisoned, as were sites of intermediate coordination (2030-2060 cm(-1)). Sites in smaller two-dimensional ensembles of Pt atoms (2060-2065 cm(-1)) were partially poisoned, as were sites for the adsorption of CO in a bridging configuration. The addition of Sn blocked the lowest coordination sites and destroyed large ensembles of Pt by a geometric dilution effect. The poisoning of other sites by coke was impeded by Sn, this effect being magnified for Cl-containing catalyst. Oxidation or oxychlorination of coked catalyst at 823 K followed by reduction completely removed coke from the catalyst surfaces.