Atomic and molecular species generated by the photolysis of aligned molecules adsorbed on crystalline solids tend to move preferentially in particular directions relative to the crystal surface .This behaviour results in surface-aligned photo-reaction [1,2] if the photogenerated species is directed towards, and reacts with, adsorbed and aligned target molecules. Previously, geometrical directionality has been inferred from the reaction product yield, the angular distribution and/or the kinetic and internal energy distributions of departing photochemically produced species [3-6]. Here we report measurements of the relative rate of the reaction between oxygen atoms (photogenerated from adsorbed and surface-aligned molecular oxygen) and carbon monoxide molecules adsorbed on either the step or the terrace sites of platinum single crystals. By using isotopically distinct carbon monoxide molecules, we are able to show that the oxidation rate at step sites is twice the oxidation rate at terrace sites. This observation suggests that the motion of the photogenerated oxygen atoms is aligned along the step edge, so that the atoms are 'aimed' at carbon monoxide molecules adsorbed at step sites.