The surface mobilities of both CO and K coadsorbed on Ru(001) were studied using laser-induced thermal desorption (LITD) techniques. The LITD measurements revealed that CO was essentially immobile on the potassium-promoted Ru(001) surface when the CO surface coverage, theta(CO), was less than the potassium coverage, Theta(K). The CO surface mobility increased dramatically when Theta(CO)>Theta(K). At Theta(K)=0.10 ML and 315 K, the CO diffusion coefficient was D-CO<4x10(-10 cm?2)/s for Theta(CO)<0.10 ML and increased to D-CO=2.5x10(-7) cm(2)/s for Theta(CO)>0.10 ML. At Theta(K)=0.25 ML and 315 K, the CO surface diffusion coefficient was D-CO<4x10(-10) cm(2)/s for Theta(CO)<0.25 ML and increased to D-CO=5X10(-9) cm(2)/s for Theta(CO)>0.25 ML. The potassium surface mobility also decreased dramatically as a function of CO coverage at both Theta(K)=0.10 ML and Theta(K)=0.25 ML. These diffusion results are consistent with a mutually stabilizing and trapping interaction between coadsorbed CO and K with a 1:1 CO:K stoichiometry. The thermal desorption spectra of both CO and K were also studied using temperature programmed desorption (TPD) technique. At Theta(K)=0.10 ML and CO coverages ranging from Theta(CO)=0.06 ML to Theta(CO)=0.42 ML, CO TPD peak temperatures were consistently higher than CO peak temperatures obtained on clean Ru(001). These TPD results indicate that CO is stabilized by K on Ru(001). In addition, the potassium TPD spectra at various CO coverages with Theta(K)=0.10 ML and Theta(K)=0.25 ML revealed that K was stabilized by coadsorbed CO. The experimental results for CO and K coadsorbed on Ru(001) argue for a mutually stabilizing, short-ranged trapping CO-K interaction with a 1:1 CO:K stoichiometry. Monte Carlo simulations based on this model for the CO:K interaction were also consistent with the coadsorbate surface diffusion results.