We studied molecular species appearing in the reactions of ethylene on a Pt(1 1 1) surface by the technique of Cs+ reactive ion scattering (Cs+ RIS). Dehydrogenation reaction of ethylene was examined for a surface temperature range of 100-800 K, and the RIS result verified the well-known sequence of forming di-sigma-bonded ethylene (-CH2-CH2-), ethylidyne (dropC-CH3), CH, and then surface carbons, as the temperature increased. In particular, the intermediate species in the conversion of ethylene to ethylidyne was closely investigated, which showed the presence of an ethylidene intermediate (=CH-CH3). In a study of H/D exchange reactions between surface C2D4 and H, we successfully identified the ethylenes in which several deuterium atoms were substituted by hydrogen (C2D4-xHx,x = 0-4), and quantitatively determined their relative populations. These examples demonstrate the ability of the Cs+ RIS method to identify small hydrocarbons and their isotope-exchanged species on surfaces. (C) 2002 Elsevier Science B.V. All rights reserved.