Density functional theory calculations have been used to study [4+2] and [2+2] cycloaddition reactions between 1,3-butadiene and the diamond(100) surface. The structure, energetics, and spectra of the products, as well as the reaction pathways and activation barriers, are discussed. The [4+2] product is energetically favored over the [2+2] product, and the barrier to [4+2] addition is lower than that to [2+2] addition, so the [4+2] product is expected to be the dominant product on the surface. Interdimer interactions influence the structure and vibrational spectrum of the [4+2] product, but have a small effect on the adsorption energy. Comparisons to experimental studies are made, and while the [4+2] product appears to be the major surface species, there is also evidence for the presence of other species.