Here we report the adsorption and decomposition behavior of dimethyl methyl-phosphonate (DMMP) on pristine and defected graphene using the first principles theory. The primary objective of this study is to highlight the importance of a vacancy defect on the adsorption mechanism of a molecule. In order to account for the weak forces involved between the molecule and graphene, we have used dispersion corrected total energy calculations along with generalized gradient approximation scheme for the exchange correlation energy. Among various possible configurations, the most stable geometry shows oxygen atom of the DMMP molecule favors to be close to the surface plane and bind with one of the unsaturated carbon atoms at the defect site. The molecule-substrate interaction energy is stronger for defected graphene than pristine graphene. The decomposition of the DMMP molecule at the vacancy site of the defected graphene has been investigated. For the dissociative adsorption, it is seen that the C-H bond of the DMMP breaks and H atom is transferred to one of the low-coordinated C-atoms at the vicinity of the defect, forming new C-H bond. In addition, the C-C bond formation between graphene and DMMP occurs. Finally, the nature of bonding and electronic structure at the interface was interpreted through site projected electronic density of states analysis. (C) 2017 Elsevier B.V. All rights reserved.