The paper proposes a new theoretical model based on local thermodynamic equilibrium enabling the prediction of gas generation during the reaction of aluminum-based thermites. We demonstrate that the model has the capability to predict the total pressure and the partial pressures of its components as a function of the reaction extent and compaction. Al/CuO, Al/Bi2O3, Al/Sb2O3, Al/MoO3, and Al/WO3 thermites are modeled and their capability to generate pressure is compared. Simulation results are also validated through dedicated experiments and show general agreement beyond the state of the art. Mechanisms underlying pressure generation are detailed. A two-stage process for the pressure increase in Al/CuO reaction, also observed experimentally, is shown to be driven by oxygen generation as produced by CuO and Cu2O vaporization through different kinetics. Comparison with experimental data stresses the issue of understanding the complex chemical processes taking place during vaporization and subsequent gas phase reactions and the need to determine their thermodynamic constants.