Using reflection-absorption infrared spectroscopy (RAIRS) and temperature-programmed reaction spectroscopy (TPRS), we have investigated the cross-coupling reaction between CH2(a) and CF3(a) on a Ag(111) surface. CH2(a) and CF3(a) are generated by thermal decomposition of adsorbed CH2I2 and CF3I. RAIRS results unambiguously demonstrate that CH2(a) inserts into the Ag-CF3 bond to produce adsorbed CF3CH2(a), which upon heating selectively undergoes beta-fluorine elimination to form CH2=CF2. Increasing the CH2(a) and CF3(a) coverage leads to the sequential insertion of CH2(a) into Ag-CF3, as evidenced by CH2CH2CF3(a) formation detected with RAIRS. Prior to the insertion reaction, the evidence favors islanding of fragments. The methylene insertion reaction is so facile that it occurs at cryogenic temperatures (120 K). Time-resolved RAIRS (TR-RAIRS) results at selected temperatures reveal an activation energy of 5.8 kJ/mol. Our results provide, for the first time, direct spectroscopic information about the mechanism and kinetics of the methylene insertion reaction.