Several Al/CuO nanocomposite reactive materials, prepared recently by arrested reactive milling, were found to exhibit a distinct low-temperature exothermic peak around 600 K seen by differential scanning calorimetry. This work is an experimental study aimed to establish whether this low-temperature reaction affects combustion and why it is observed in some but not all materials with identical compositions. The peak is only observed when aluminum is initially separately milled in acetonitrile. Electron microscopy showed resulting composite powders to be porous, unlike fully dense powders obtained without premilling aluminum. The as-prepared and partially reacted powders recovered after heating just past the peak to 650 K were examined using high-angle annular dark-field scanning transmission electron microscopy and electron energy loss spectroscopy. Additionally, X-ray diffraction measurements were performed for all materials. Finally, the reactive powders were ignited using plasma and shock generated by an electrostatic discharge. Results show that the low-temperature exothermic peak is associated with a redox reaction pathway involving release of oxygen by CuO that is not in direct contact with Al and freemolecular transport of that oxygen to the nearby surface of Al. This reaction pathway inhibits the formation of Al/Cu intermetallic phases. Instead, nanometer-scale metallic Cu particles are formed due to CuO reduction. It is also found that this reaction pathway accelerates ignition of reactive powders, which in turn leads to a higher burn rate of aerosolized powder.