The adsorption and reactions of vinyl bromide and vinyl iodide on a Cu(100) surface have been studied by temperature-programmed desorption in conjunction with near-edge X-ray absorption fine structure (NEXAFS) and work function change measurements. Vinyl bromide adsorbs molecularly on the surface at 100 K. The polarization dependence of the pi*(c=c) resonance indicates that the molecules lie with their pi bond within 28 +/- 5 degrees of parallel to the surface. Upon healing, both vinyl bromide and vinyl iodide decompose to generate surface vinyl groups, which adopt a tilted orientation on the surface. Both the molecular halides and the surface vinyl groups show a splitting of the pi*(c=c) NEXAFS resonance due to the inequivalence of the carbon atoms in these species. The position of the sigma*(c-c) shape resonances for these species indicates little change (<0.05 Angstrom) in C=C bond length due to adsorption and dissociation to form vinyl groups. Chemical displacement studies show that the C-Br bond cleavage in vinyl bromide occurs at 160 K. This dissociation temperature is confirmed by complementary NEXAFS and work function change measurement results. At 250 K, vinyl groups couple to yield 1,3-butadiene with 100% selectivity.