The adsorption of methyl chloride (CH3Cl) on Si(001) surfaces is studied using C,first-principles calculations based on gradient-corrected density-functional theory (DFT-GGA) and ultrasoft pseudopotentials. The energetically most favored structure for the adsorption of a single molecule is characterized by the dissociation of methyl chloride into CH3 and Cl fragments, which bond to the same Si dimer. The plausible interface structures are examined with respect to their band structures, surface dipoles, and charge-transfer characteristics. A remarkable sensitivity of the surface electronic properties with respect to the details of the bonding is found.