The selectivity for ethylene production from the reactions of ethanol on oxygen-covered and sulfur-covered Mo(110) is significantly increased from that on the clean surface. The presence of these surface modifiers inhibits the activity for nonselective decomposition, favoring ethylene elimination; On both the oxygen and sulfur overlayers, ethoxide decomposition yields ethylene and H-2 as the major gaseous products. Water is also evolved on the oxygen-covered surface. The total amount of ethanol decomposition is＇ significantly reduced when sulfur is present such that on the 0.5 ML sulfur overlayer it is only 9% of that on clean Mo(110). The decrease in the total amount of decomposition due to sulfur is attributed to site blocking by sulfur binding in high coordination sites. Surprisingly, oxygen bound to high coordination sites does not significantly inhibit irreversible ethanol reaction until more than half of the sites are occupied. The total amount of ethanol that irreversibly reacts remains nearly constant up to oxygen coverages of 0.5 ML. We propose that the sustained reactivity of the oxygen overlayers is due to the small diameter of oxygen relative to S, so that high coordination sites adjacent to those occupied by oxygen are available for ethoxide coordination. The formation of chemisorbed OH from hydrogen transfer to chemisorbed oxygen on Mo(110) may also play a role in reducing site blocking by oxygen, because the charge and diameter of OH will be smaller than for oxygen itself and because adsorbed OH may move to a slightly different coordination site; On more highly oxidized surfaces; where subsurface and terminal sites (Mo=O) are populated, the amount of ethanol decomposition is significantly reduced. In particular, we show that the Mo=O species does not promote ethanol reaction. We attribute the lack of ethanol decomposition on the more highly oxidized surfaces to the lack of facility for OH and water formation and to the absence of vacant coordination sites for the ethoxide. The reactivity of the oxidized Mo(110) is contrasted to MoO3 catalysts and differences are ascribed to the different oxidation states of the materials.