Using a high-pressure scanning tunneling microscope we monitored the coadsorption of hydrogen, ethylene, and carbon dioxide on platinum(111) and rhodium(111) crystal faces in the mtorr pressure range at 300 K in equilibrium with the gas phase. During the catalytic hydrogenation of ethylene to ethane in the absence of CO the metal surfaces are covered by an adsorbate layer that is very mobile on the time scale of STM imaging. We found that the addition of CO poisons the hydrogenation reaction and induces ordered structures on the single-crystal surfaces. Several ordered structures were observed upon CO addition to the surfaces precovered with hydrogen and ethylene: a rotated (root19 x root19)R23.4degrees on Pt(111), and domains of c(4 x 2)-CO + C2H3, previously unobserved (4 x 2)-CO + 3C(2)H(3), and (2 x 2)-3CO on Rh(111). A mechanism for CO poisoning of ethylene hydrogenation on the metal single crystals was proposed, in which CO blocks surface metal sites and reduces adsorbate mobility to limit adsorption and reaction rate of ethylene and hydrogen.