Rocket engines, gas turbines, HCCI engines, and other such combustion devices frequently exceed the critical pressure of the fuel or the oxidizer. Modeling of combustion processes at high-pressure operating condition is required to determine the reaction rates based on which chemical kinetic models are developed. The current need is to focus on the transfer from low pressure to high-pressure conditions as this can have a significant effect on the chemistry as well as the reaction rates. The ReaxFF reactive force field method is a computationally feasible method used to study the combustion kinetics of fuels and fuel mixtures at supercritical condition. In this chapter, ReaxFF-MD is used to investigate the effect of a highly reactive fuel on the properties of a less reactive fuel at different levels of concentration, temperature, and density/pressure. The activation energies, based on Arrhenius-type rate laws, are compared with those from Continuum simulations and the limitations of the latter has been elaborated on. The study reveals a pressure/temperature regime and mixing conditions, where simple first-order kinetics-based Arrhenius-type relations cannot be applied. The reason can be attributed to different initial reaction mechanisms and product distributions of the two fuels considered. These results indicate how ReaxFF-based molecular dynamics simulations can provide significant atomistic insights on the combustion properties of fuel mixtures at supercritical conditions, where experiments are difficult to perform.