The adsorption of cyclopentadiene and its dimer form, dicyclopentadiene, on the Si(100)-2x1 surface has been investigated using polarized multiple internal reflection Fourier transform infrared (MLR-FTIR) spectroscopy together with ab initio quantum chemistry calculations. The results show that cyclopentadiene adsorbs at room temperature to form a [4 + 2] cycloaddition product and at saturation covers less than two-thirds of the surface. Dicyclopentadiene adsorbs intact, most likely by a [2 + 2] cycloaddition reaction, and is estimated to adsorb onto 30-40% more Si=Si dimer sites than cyclopentadiene despite its larger size. Dicyclopentadiene is not found to react via a surface-catalyzed retro-Diels-alder pathway whereby it splits into cyclopentadiene while chemisorbing, even though this pathway is theoretically predicted to be much more thermodynamically favored.