The kinetics and thermodynamics of thermal decomposition of naphthalene and several mono- and dimethylnaphathalene derivatives have been established through reactive molecular dynamics simulations using ReaxFF. These results were compared to previous theoretical and experimental studies and also compared to density functional theory calculation results. This work demonstrates that the kinetics and thermodynamics of the initial activation reaction is directly impacted by the position and number of methyl substituents. Subsequently, the activated molecules react to form either small organic or large char molecules. The char formation mechanism is shown to occur in three steps: activation, dimerization/trimerization, and condensation. Finally, temperature effects on the char formation reaction were also studied.