The chemisorption of toluene on the Si(100) surface is studied using first principles and semiempirical methods. We find that the most stable configuration is a dissociated one, in which a C-H bond of the methyl group is cleaved and the loose hydrogen is bonded to the silicon surface; a detailed analysis based on the use of the maximally localized Wannier functions indicates that this process can be described as a proton abstraction reaction. Several other stable geometries involving both undissociated and dissociated toluene molecules have been found and studied. Although most of them resemble those of benzene on Si(100), some differences have been detected, which mainly reflect the role of the electron-donating, substituent methyl group of toluene. Possible reaction pathways leading from one stable adsorption configuration to another and to the dissociation of the toluene molecule on the Si surface have been investigated; in particular the one involving the abstraction of a proton from the methyl group has been found to be activated, with an energy barrier of about 1.4 eV. Our results are compared with recent experimental measurements.