We analyzed thermite reactions in composites comprised of Al and CuO nanoparticles (NPs) and investigated pressure change and pressurization rate as a function of composition. Although the stoichiometric ratio for the thermite reaction between Al and CuO is 2:3, our experiments show that the maximum pressurization rate occurs in Al-rich thermite composites. Transmission electron microscopy analysis of the reaction products revealed that a considerable number of Al NPs are coated with Cu layer. We attribute the formation of Cu layer on the surface of Al NPs during the reaction to the off-stoichiometry, since the Cu layer renders Al NPs relatively less reactive. Based on our analysis of the experimental data, we propose a mechanism in which gasified Cu grows into Cu layer, as the gas-phase Cu species travels from reaction sites to unreacted Al NP surface. Analysis on characteristic timescale of heat and mass transfer reveals that condensation of Cu on Al NPs occurs faster than the overall thermite reaction.