As a part of a continuing investigation of acetylene reactions on tungsten surfaces covered by ultrathin metal films, the chemistry of acetylene on clean and carbide-modified W(211) surfaces has been investigated using high-resolution electron energy-loss spectroscopy (HREELS) and temperature-programmed desorption (TPD) measurements. Acetylene decomposes between 100 and 450 K on the clean W(211) surface, ultimately forming carbidic carbon and gaseous hydrogen. During this decomposition process, a mixture of different hydrocarbon fragments are formed, most likely vinylidene (CCH2) pr vinyl (HCCH2) and acetylide (CCH). On the carbide-modified W(211) surface, acetylene also decomposes to produce carbon and gaseous hydrogen. However, compared to clean W(211), the CH bond activation is suppressed on the carbide-modified W surface, as evidenced by the broadening and significant, shifting ai the H-2 TPD features toward higher temperatures as well as by the HREEL spectra monitoring the thermal decomposition of acetylene on the carbide-modified W(211) surface.