We investigated the adsorption states and the reaction of vinyl bromide (CH2=CHBr) on Si(100)c(4x2), using high-resolution electron energy loss spectroscopy. We found that the chemisorption of this molecule occurs via a precursor state and a vinyl bromide molecule is di-sigma bonded to the Si dimer on Si(100)c(4x2) in the chemisorption state. The precursor species are observed in the temperature range from 58 to 90 K. Judging from the vibrational spectra, we conclude that the structure of the precursor state is a three-atom intermediate state ( pi complex). The activation energy and the preexponential factor of the reaction from the precursor state to the chemisorption state are estimated to be 27.3 kJ/mol (283 meV) and 1.5 x 10(13) s(-1), respectively. The activation energy decreases with increasing the vinyl bromide coverage owing to the steric repulsion.