The effect of coadsorbed CO on potassium surface diffusion on Ru(001) has been studied using coverage grating-optical second harmonic diffraction method. The enhancement of the first-order diffraction peak at 0.06 ML of CO coadsorption coverage demonstrates the very strong electronic interaction between these coadsorbates. At an initial potassium coverage of 1.0 ML, the activation energy for the diffusion of K increases from 2.7 kcal/mol on the clean ruthenium surface to 17.0 kcal/mol in the presence of 0.06 ML CO. At 280 K, the diffusion rate coefficient drops from D = 6.4 x 10(-10) cm(2)/s on clean ruthenium to D = 1.5 x 10(-14) cm(2)/s as a result of CO coadsorption. CO and K stabilize each other upon the formation of K-y-COx surface complexes that increase the apparent activation energy for potassium diffusion, thereby slowing its diffusion rate.