The bistable behavior of the O-2 + H-2 reaction on Rh(110) is modified by the presence of coadsorbed potassium. Reaction fronts transporting potassium and the development of stationary Turing-like patterns have been observed. A realistic mathematical model is presented which reproduces qualitatively correct and, to a large part, even quantitatively correct the experimental results. Key factors of the model are the strong chemical affinity between coadsorbed oxygen and potassium, a reduced mobility of potassium on the oxygen covered surface, and a strongly reduced reactivity of oxygen toward hydrogen in the presence of coadsorbed potassium.