This paper investigates the thermochemistry of the heterogeneous Al/CuO thermite reaction through density functional theory calculations. We examine the interactions of atomic Al, Cu, O, as well as O2, AlO, Al2O, AlO2, Al2O2 molecular species, with Al(111), Cu(111), and Al2O3 (γ and amorphous) surfaces, all of which being condensed phase products during the thermite reaction. Al(111) exhibits a very high reactivity, characterized by adsorption energies ranging from 3 to 5.3 eV for atomic Al, Cu, O, and from 4 to 9.5 eV for all molecular species. This reactivity is attributed to barrierless molecular decomposition, followed by the spatial spreading of adsorbate species across the surface facilitated by hot adatom migration processes. The Al 2 O 3 surface also exhibits extremely high reactivity, with adsorption energies of 4.5 and 9.4 eV for atomic Cu and Al, respectively. Additionally, absorption energies range from 7 to 15 eV for condensation of AlxOy suboxides. Al-rich suboxides, namely Al 2 O and Al 2 O 2 , show the greatest adsorption energy with -15.05 eV for Al 2 O, against 6.52 eV for AlO 2 . In contrast, O and O 2 exhibit no reactivity on Al 2 O 3 surfaces exhibiting oxidation states being superior or equal to Al III . Finally, Cu(111) surface exhibits much lower reactivity compared to Al(111) and Al 2 O 3 , with absorption energies ranging from 2 to 3.5 eV for Al, O and Cu atoms. Although energetic, molecular AlxOy suboxides show non dissociative adsorption on Cu(111). This findings point to different modes of oxide nucleation on these surfaces, pleading for planar nucleation and growth onto Al(111), while being more difficult and localised onto Cu(111). They renew our understanding of the thermite reaction chemistry, quantitatively differentiating the various type of heterogeneous reactions and their implication on the overall reaction. They also provide valuable data for higher-level diphasic simulations of the computational fluid dynamics, aiming to achieve predictive capability.