The decomposition of ethylene was utilized as a probe reaction to determine the chemical properties of carbide-modified W(110) and W(111) using high-resolution electron energy loss spectroscopy (HREELS) and temperature-programmed desorption (TPD) measurements. Ethylene adsorbed via a di-sigma configuration on both clean and carbide-modified W(110). Most of the di-sigma bonded ethylene converted to ethylidyne species by 200 K. In contrast, ethylene decomposed to form vinylidene and acetylide intermediates without forming ethylidyne at 90 K and 200 K on clean and carbide-modified W(111). Thermal desorption results showed clean W(111) to be twice as active as W(110) toward the decomposition of ethylene. The presence of the carbide overlayer reduced the activity of the W(111) surface by similar to50%, whereas carbide-modification only reduced the activity of the W(110) surface by similar to30%. This study showed that the reaction pathways of ethylene on carbide-modified surfaces were strongly dependent on the structure of the metal substrate.