A model based uniquely on condensed phase reactions coupled with the thermal equation is developed to study the initiation and early stage of the redox reaction in Al/CuO nanothermites. It considers the effect of a wetting contact angle between Al and CuO particles, which may be induced by sintering mechanisms and/or the synthesis method. In order to validate the model, two published experiments are reproduced in silico. Results provide the first quantification of: (i) how sintering affects the initiation of Al/CuO nanoparticle mixtures, depending on experimental conditions, (ii) the extent to which condensed phase mechanisms dominate gas-mediated reactions in the initiation process, two subjects that have been highly debated in the literature. It was found that initiation appears more strongly affected by sintering when particles are exposed to an ultra-short and intense heat pulse (∼1011 K s−1) than those exposed to a lower heating rate (∼105 K s−1). Additionally, calculations show that sintering may cause a drastic decrease in the initiation delay (down to the ns regime) when using CuO nanoparticles below 50 nm in diameter that can be brought to melting temperature through optical absorption. Finally, the role of gas-surface versus condensed phase reactions in the Al/CuO initiation process is evaluated theoretically. Initiation through condensed phase reactions, while slightly faster and more efficient, exhibits a comparable timescale (∼1–2 ms) to initiation through gas-surface reactions, providing clear evidence for the contribution of both during the initiation phase