The desorption and reaction of cyclopentadiene on Cu(100) were studied by temperature-programmed desorption and Auger electron spectroscopy. The dehydrogenation of cyclopentadiene is significant at low exposures (less than or equal to 0.8 L), producing cyclopentadienyl (C-5-H-5) species as a reaction intermediate at less than or equal to 335 K. The C5H5 groups are stable on the Cu(100) surface up to similar to 600 K. At higher temperatures, cyclopentadienyl species undergo a self-hydrogenation reaction, with two-thirds of the C5H5 groups obtaining H from the others, and desorb from the surface as cyclopentadiene (C5H6) at similar to 620 K. The remaining one-third of the C5H5 groups dehydrogenates to form hydrocarbon fragments that can be best described as C5H3 species, which further decompose to produce residue surface carbon and molecular H-2 at similar to 700 K. The implication of the current work to the previous observation of carbon contamination in the chemical vapor deposition of Cu is also discussed.