We have used ab initio calculations to study the structural and electronic properties of two simple hydrocarbons, C 2 H 4 and C 2 H 2 , adsorbed on the Si(001) surface. The hydrocarbons adsorb above surface dimers, saturating the dangling bonds and in the process weakening the carbon-carbon bond of the molecule. We found that the σ-bond of the silicon dimer remains intact but may be broken by post-hydrogenation of the surface. This resolves an apparent conflict between previous calculations and recent experiments.