Time-of-flight secondary ion mass spectrometry (TOF-SIMS) has been employed to examine the chemistry of highly dispersed Pt on alumina. In particular, we have characterized various negatively charged Pt cluster ions emitted from Pt powder and fresh and discharged 0.6% Pt/Al2O3 after bombardment with 25-keV Ga+ ions. For Pt powder, PtO- and PtO2- dominate the spectrum of negative ions, which indicates the presence of surface Pt oxides. The most important observation, however, is that PtCl-, PtClO-, and PtCl2- cluster ions dominate the negative ion spectrum of 0.6% Pt/Al2O3. These chlorinated Pt ions provide direct spectroscopic evidence that some Cl atoms are still bound to Pt after high-temperature oxidation and reduction.