The structures and energetics of organic molecules adsorbed onto clean and H-passivated Si (001) - (2×1) surfaces have been calculated using density functional theory. For benzene adsorbed on the clean Si surface the tight-bridge structure was found to be stable and the butterfly structure metastable. Both carbonic acid, H2 C O3, and propane, C3 H8, dissociate on contact with the surface. Passivation of the Si surface with H atoms has a dramatic effect on the surface properties. The passivated surface is very inert and the binding energy of all the molecules is very weak.