The adsorption of methanol, formaldehyde and formic acid on the Si(100)-2 1 surface have been investigated by means of first-principles density functional cluster model calculations and ab initio ONIOM calculations. The dissociative adsorption of methanol on the Si(100) surface takes place readily, giving rise to Si-OCH3 and Si-H surface species. The reaction, occurring barrierlessly via a stable chemisorbed state and the transition state for dissociation, is highly exothermic. The chemisorption of formaldehyde on the Si(100) surface is also barrierless and exothermic with the formation of a 4-member ring -SiCOSi- surface species. This result indicates that the carbonyl (C=O) group can undergo cycloaddition onto the Si dimer on the Si(100) surface. The dissociative chemisorption of formic acid occurs readily on the Si(100) surface with the formation of unidentate formate surface species and H adatoms. Its exothermicity is higher than 60 kcal mol-1. The vibrational frequencies of the surface species produced by the chemisorption of the three C1 molecules have been calculated and compared with the available experimental data.