Surface-aligned photo-oxidation of CO with O-2 was demonstrated on two stepped surfaces, Pt(335) and Pt(779). UV light photolysis of O-2, localized exclusively on step sites, generates reactive oxygen atoms which oxidize preferentially the step-site CO. This effect was observed by monitoring labeled-CO consumption from step and terrace sites by reflection-absorption infrared spectroscopy, and also by monitoring labeled-CO2 desorbed in the gas phase from step and terrace sites by mass spectrometry. Photo-oxidation of step-CO was found to be about 2 times more efficient than photo-oxidation of terrace-CO, irrespective of the surface step density. This effect was attributed to the initial alignment of the O-2 intramolecular axis on stepped Pt surfaces, parallel to the step edge.