We present an ab initio investigation of the structure and thermochemistry of the products of the chemisorption of 1,3-butadiene on the Ge/Si(100)-2 x 1 surface. The surface was modeled using a Si cluster with one dimer and Ge substitution for surface Si to study the effects of Ge on the cycloaddition. Calculations were performed using the Becke3LYP hybrid density functional theory method. Both the [4 + 2] Diels-Alder and the [2 + 2] cycloaddition products are energetically stable on all the Ge/Si(100)-2 x 1 surfaces investigated. The [4 + 2] Diels-Alder products on the Si-Si, Si-Ge, and Ge-Ge dimers are more stable than the corresponding [2 + 2] cycloaddition product due to ring strain. The binding energies of all cycloaddition reaction products decrease with increasing Ge composition on dimers, which can be explained by differences in bond strength. The structures and energetics will be explained and discussed.