Density functional theory calculations (B3LYP and BH&HLYP functionals) of the potential energy surface have been performed to investigate the mechanisms of decalin breakdown, and the Rice-Ramsperger-Kassel-Marcus and transition state theory methods have been used to compute the high-pressure limit thermal rate constants for the new reaction pathways. The new pathways connect decalin to five primary monoaromatic species: benzene, toluene, styrene, ethylbenzene, and xylene. The reactions used for the new routes are carbon-carbon bond cleavage reaction, dissociation reaction, and hydrogen abstraction and addition reactions. A kinetic analysis was performed for pyrolytic conditions, and benzene, toluene, and xylene were identified as major products.