The paper is a theoretical exploration of complex Al/CuO thermite combustion processes, using a zero-dimensional (0D) model which integrates both condensed phase and gas phase reactions, and considers all thermodynamic stable molecular or atomic species identified during the Al+CuO reaction. We found that the particle size mainly influences the reaction kinetics and pressure development. Thermite with nano-sized particles (nanothermites) burns ~10 times faster than the same thermite with micron-sized particles (microthermites). This is due to the fact that the thermite reaction occurs mainly in condensed phase, i.e. in the melted Al phase, as all gaseous oxygens released by the CuO decomposition are spontaneously absorbed on the huge specific surface area of metallic Al. As a consequence, the pressure development in nanothermites follows the thermite chemical reaction, the gas phase is mostly composed of a metal vapor (mostly Cu and Al), Al suboxides, but is free of molecular oxygen. In contrast, when dealing with microthermites, an oxygen pressure peak occurs prior to the thermite reaction due to the gaseous O2 released by the early CuO decomposition, that cannot be absorbed on the Al particles surface in real time. The powder stoichiometry greatly impacts the final pressure. Al lean thermites generate a higher final pressure (× 3) than stoichiometric and Al rich mixtures, due to unreacted gaseous oxygen which remains in the gas phase after the full consumption of the metallic Al.