Electron irradiation on adsorbed methyl groups on Cu(110) at 90 K to produce coadsorbed methylene groups is observed by HREELS. The cross-sections for CH2(ads) production and CH3(ads) desorption are determined at 15 eV e-beam energy. In TPD measurements, CH2(ads) groups mainly react with CH3(ads) to generate gaseous ethylene, propylene, and butene with desorption peaks centered all at 310 K independent of the CH2(ads) surface concentration. The product ratio of C3H6/C2H4 is linearly proportional to the ratio of CH2(ads)/CH3(ads). The results suggest that aggregation of the adsorbates and close proximity of the reactants are critical for the chain propagation reaction. (C) 2002 Published by Elsevier Science B.V.