The adsorbate resulting from the potentiostatic, catalytic decomposition of methanol (0.1 M CH3OH in 0.5 M H2SO4) on a platinum black electrode has been studied via C-13 nuclear magnetic resonance spectroscopy, at open cell potential. Cyclic voltammetry results indicate that coverage is a function of electrodecomposition time and potential. The spin-spin relaxation time T-2 is dependent on surface coverage and ranges from similar to 3 to similar to 1.8 ms at coverages ranging from 0.3 to 0.75 ML, due to the increased effectiveness of C-13-C-13 dipolar interactions at high coverage. At 0.5 ML, the temperature dependence of the (CO)-C-13 T-2 (in a (H2O)-H-2-exchanged electrolyte system) has been determined from 80 to 250 K. There is a well-defined peak in relaxation rate at similar to 170 K which can be modeled using a simple diffusional model having an activation energy of 7.9 +/- 2.0 kcal/mol. Spin-lattice relaxation results from 10 to 250 K reveal Korringa behavior, with a T1T product (and Knight shift) that is independent of surface coverage, and has the same value for the electrochemical adsorbate as gas phase CO adsorbed on Pt. The similarity in T1T, T-2, Knight shift, and activation energy for surface diffusion are in general accord with values previously measured in gas phase heterogeneous catalyst systems and strongly support the idea of primarily on top CO with C-down, on one major type of surface site.