We report a comparative study of unsteady behavior of CO oxidation over silica-supported platinum and Pt(100). Experiments were performed in a reaction cell allowing the first direct comparison between both catalyst types under the same conditions at pressures between 10 and 100 mTorr. The reaction is examined with mass spectrometry, time-resolved FT-IIE spectroscopy, and ellipsomicroscopy for surface imaging (EMSI). For the supported catalyst, our measurements concentrated on the reaction rate as a function of control parameters pressure and temperature and time-resolved FT-IR, EMSI was used to follow spatio-temporal pattern formation on the Pt(100) single crystal. The parameter space (partial pressures of the reactants, catalyst temperature, pumping speed of the reactor) for unsteady behavior of the CO oxidation is much wider for the supported catalyst. In certain parameter ranges, spatio-temporal pattern formation governs the unsteady behavior of the single crystal, while for other parameters, very regular oscillations with no pattern formation are observed. Time-resolved FTIR spectroscopy was used to follow the CO coverages under reaction conditions, which stayed below 15% of the maximum coverage