The influence of co-adsorbates on the methylene coupling reaction on Ag(1 1 1) was investigated by means of reflection-absorption infrared spectroscopy (RAIRS) and temperature-programmed reaction spectroscopy (TPRS). Generated by thermal decomposition of adsorbed CH2I2, CH2(a) selectively self-couples to form ethylene. The desorption of ethylene resulted from methylene coupling reaction shows an interesting coverage-dependent, which on basis of the RAIRS and TPD results, is attributed to both the existence of co-adsorbates on the surface and the coverage-dependent C-1 rupture of CH2I2 (a). The reaction kinetics of methylene coupling reaction is also affected by the co-existence of CF3I(a) on the surface. It is the migration and/or assembly of methylene on the surface, not the actual methylene coupling reaction, which is influenced by the co-adsorbates. (c) 2005 Elsevier B.V. All rights reserved.