The interaction of CO with K-free and K-covered Mo2C/Mo(100) surfaces has been investigated by HREELS, AES, and TPD. CO adsorbed molecularly on the clean Mo2C/Mo(100) surface at 140 K. A part of the adsorbed CO desorbed from the saturated adlayer with T-p = 330 K, which was accompanied by dissociation of the molecule at 300-350 K. The C and O adatoms formed recombined at higher temperatures, giving a CO desorption peak at 960 K. Preadsorbed potassium at intermediate coverages (Theta (K) = 0.5-0.8 ML) led to a considerable weakening of the C-O bond even at 140 K, as evidenced by the HREELS feature at 1320-1375 cm(-1). The appearance of this loss suggested a short-range interaction, while the development of losses at 1670-1685 and 1860 cm(-1) is attributed to long-range interaction in the coadsorbed layer. Potassium adatoms decreased the amount of CO desorbed and converted it into a more reactive form, which-instead of desorption-underwent dissociation at 350-550 K. The recombinative desorption of CO. occurred at 970 K; its amount was 3 times higher than that measured for K-free Mo2C. Potassium was stabilized in the adsorbed layer mainly by O adatoms arising from the decomposition of CO.