This thesis is concerned with the numerical simulation of mixtures of chemically reacting fluids at high pressures. Mixture rules are established, accommodating components that do not follow the thermally perfect Equation of State (EoS), and including formulas for the fugacities. Particular attention has been given to the species EoS originally developed by Hirschfelder, Buehler, McGee and Sutton. The Law of Mass Action that governs chemical equilibrium of mixtures of fluids is presented. The common assumption of neglecting fugacities contributions when calculating the equilibrium composition is assessed by comparing complete thermochemical predictions with those obtained by reduced models. For air mixtures at high pressures and moderate temperatures, species thermally imperfect EoS effects are significant. However, the contribution of fugacities to the Law of Mass Action is found to be small, at least for mixtures of super-critical dissociating gases. Similar conclusions apply to combustion mixtures (involving hydrogen and oxygen).