We have used NMR to study the effects of adding alkali metals (Na, K, Rb, or Cs) to silica supported platinum clusters. Alkali metals are frequently added to metal catalysts, and to other catalysts as well, to serve as ＇promoters＇. We have also examined the effect of the alkali metals on adsorbed CO. The properties of the alkali metal itself were studied in samples containing Na. By performing a Na-23-C-13 double resonance with adsorbed CO, we determined that at least 26% of the sodium in the samples is on the platinum clusters. The Na-23 line shows no indication of a Knight shift and the Na-23 spin-lattice relaxation time varies as 1/T-2, suggesting that the sodium orbitals mix with the platinum conduction band to a much lesser degree than do those of adsorbed CO. The NMR results lead us to conclude that the Na forms three-dimensional structures on the Pt, most likely small Na2O clusters with sizes approaching molecular dimensions. We observed the effect of the alkali metals Na, K, Rb, and Cs on the NMR properties of adsorbed CO. In all cases the effects are small. The CO line is narrowed and shifted to higher frequency in the presence of the alkali metal while the CO spin lattice relaxation is slowed. The effect of the alkali metal is greater in the samples with the heavier alkali metals K, Rb and Cs. In the case of CO with Rb, for which the effect of the alkali metal is greatest, a modest change of about 10-15% in the parameters used to describe the NMR results is adequate to account for the effect of the Rb.