The bonding and reactivity of allyl groups (CH2CHCH2) on a Cu (100) surface are studied by temperature-programmed desorption and near edge X-ray absorption fine structure measurements. Surface allyl groups are generated by dissociative adsorption of allyl chloride at 110 K and/or a coupling reaction of surface methylene (CH2) and vinyl (C2H5) groups. Allyl groups on a Cu(100) surface coordinate as a nearly flat-lying pi-allyl species. Allyl radical ejection is the dominant reaction pathway upon heating the Cu(100) surface, which is drastically different from the behavior of allyl on Ag(111) and Al(100) surfaces, where allyl groups couple to form 1,5-hexadiene and disproportionate to produce propylene and an unidentified polymeric species, respectively. On a Cu(100) surface precovered with hydrogen atoms, allyl groups are scavenged by hydrogen atoms to produce propylene at similar to 250 K.