The bonding and reactivity of ethylene on hydrogen presaturated Fe(100) has been investigated using temperature programmed reaction spectroscopy (TPRS) and electron energy loss spectroscopy (EELS). Hydrogen preadsorbed on Fe(100) limits the extent of electron back-donation to coadsorbed ethylene, resulting in weak rehybridization relative to adsorption on clean Fe(100). Adsorption at 100 K leads to rehybridization intermediate between the sp(2) gas phase species and the nearly sp(3) di-sigma bound ethylene on clean Fe(100), as indicated by pi sigma parameters of 0.58 (0.53 for C2D4). Migratory insertion of coadsorbed hydrogen into this adsorbed ethylene occurs below 170 K; vibrational spectroscopy positively identifies the intermediate formed as surface ethyl groups.