The thermal decomposition pathways of isobutene and I-butene on both Mo(110) and (4 x 4)-C/Mo(110) surfaces have been studied using high-resolution electron energy loss spectroscopy (HREELS) in order to highlight the substantially different activities of these two surfaces towards the cleavage of C-H and C-C bonds. On clean Mo(110), the CH2 group of isobutene decomposes upon heating to 150 K, producing either a phi/pi-bonded isobutenylidene [(CH3)(2)CCH] species or a 1,1-di-phi/pi-bonded isobutenyl [(CH3)(2)CC] species. Upon further heating, extensive C-H bond scission occurs to form hydrocarbon fragments which do not contain CH3 or CH2 groups, but appear to have largely intact carbon skeletons. By contrast, isobutene is molecularly adsorbed on the carbide-modified surface at 150 K. Further heating produces isobutylidyne [(CH3)(2)HCC] by 300 K, which subsequently decomposes via C-C bond scission to generate surface methyl groups. The different activation sequence of the C-H and C-C bonds of isobutene on clean and carbide-modified Mo(110) surfaces is also qualitatively confirmed by comparative studies of 1-butene on the two surfaces.