Graphene growth from chemical vapor deposition (CVD) commonly employs methane (CH4) as carbon precursor but requires temperatures in excess of 900°C. Aromatic hydrocarbons, especially toluene (C7H8), may lower the growth temperatures well below 600°C, while preserving graphene quality; however, molecular decomposition reactions and early nucleation steps of CVD graphene using toluene are not known in details. We investigate the decomposition steps of toluene adsorbed onto Cu(111) and c(4x2)-reconstructed Si(100) surfaces through DFT calculations. The geometry and energy of toluene and most likely decomposition by-products were analyzed for various adsorbate structural configurations. Early decomposition reactions were studied through investigation of minimum energy pathways and transition states. Low activation energies were found for H removal from the methyl group of toluene …