A comparative investigation of the surface reaction of ethylene with clean Mo(110) and carbide-modified Mo(110) has been carried out using high-resolution electron energy loss spectroscopy (HREELS) and temperature programmed desorption (TPD). As typically observed for early transition metals, the clean Mo(110) surface interacts very strongly with ethylene, as indicated by the decomposition of ethylene to produce C 2H2 surface species at temperatures as low as 80 K. The surface acetylene species further decompose to atomic carbon and hydrogen at higher temperatures. The strong reactivity of the Mo(110) surface can be modified by the formation of carbide. The surface reactivity is modified in such a way that the reaction mechanism of ethylene on C/Mo(110) is very similar to those typically observed on Pt-group metal surfaces:  At 80 K, ethylene molecules bond to the C/Mo(110) surface in the di-σ bonded configuration; a new surface reaction intermediate, which can be best described as ethylidyne species, is detected in the temperature range of 260−350 K. In addition, the interaction of ethylene with oxygen-modified Mo(110) is also compared to reveal the different modification effects of carbon and oxygen adatoms on the reactivities of Mo(110). The oxygen-modified Mo(110) surface is found to be inert toward the decomposition of ethylene, as indicated by the formation of weakly adsorbed π-bonded ethylene species at 80 K and by the reversible molecular desorption at higher temperatures.