The oxidation reactions of CH3 and CH2 fragments on MoO3(100) were studied following the changes in the electronic structure and the bonding character of the bonds between the adsorbed fragments and the MoO3 surface atoms. The adsorption energy for these fragments was computed using a methodology based on the atomic superposition and electron delocalization molecular orbital theory. The electronic structure was analyzed by the local density of states concept. A detailed picture of the bonding between fragments and the surface is given by examining the overlap population. In the homolytic mechanism each C-H bond breakage occurs before the corresponding hydrocarbon fragment reaches the energy barrier and the O-H is formed after surmounting this barrier. A strong C-O chemical bond is established as a consequence of the last Il abstraction. On the other hand, in the heterolytic mechanism the C-H breaking is accompanied by a temporarily weakening of a second C-H bond and the formation of an increasingly strong C-Mo bond. In the last H abstraction of methane decomposition the C-H bond is broken only in the final step. While on the layer of MoO3 exposing mainly O atoms, the interactions between the CH3 or the CH2 fragment and the surface O atom are negligible; on the layer of MoO3 exposing Mo atoms, the molecular orbitals of these fragments show an important hybridization due to the significant C-Mo chemical interaction. (C) 2000 Academic Press.