The adsorption of C 2H 4 on the Mo(110) surface near room temperature and at 1100 K was studied with Auger electron spectroscopy, low energy electron diffraction, thermal desorption spectroscopy and electron energy loss spectroscopy. At both temperatures, adsorption saturated after exposures of about 6 langmuirs, (1 L = 10 -6 Torr · s), indicating the formation of a single layer which passivates the surface to further C 2H 4 adsorption. During thermal desorption, following adsorption near room temperature, C 2H 4 dissociates completely by 750 K to surface carbon and desorbed H 2; no hydrocarbon desorption was observed. For adsorption above 750 K, the hydrogen desorbs at once leaving only carbon on the surface. No ordered overlayer was found at 240 K or during subsequent heating; however, following a vacuum anneal after saturation at either 800 or 1100 K a Mo(110)-(4 × 4)-C structure was formed. The ratio of the number of carbon to molybdenum atoms on this surface was C : Mo = 0.37 ± 0.06, assuming unity sticking probability for C 2H 4 on the clean surface. Thermal indiffusion of carbon from the (4 × 4) surface begins at about 1400 K and becomes very rapid by 1600 K. When the C : Mo ratio fell to about 0.2, a second previously unreported structure, Mo(110)-( -4433)-C formed. These overlayers probably result from chemisorbed carbon rather than from the formation of a true surface carbide.