Nanocomposite materials with reactive components are of interest for many applications in pyrotechnics, explosives, and propellants. Several such materials have been recently prepared by Arrested Reactive Milling (ARM), a method based on mechanical milling of μm-sized component powders to form μm-sized composite particles in which the components are mixed on a scale of 100 nm or finer. The temperature at which the milling is performed affects significantly both the rate at which the material is refined and the final properties of the product. In the present paper we report on an effort to prepare Al-CuO reactive nanocomposites at cryogenic temperatures. A SPEX Certiprep 6815 Freezer/Mill was used to prepare the nanocomposites with aluminum-rich compositions from μm-sized component powders. The material was processed in steel vials using steel balls of different sizes as milling medium. The number and dimensions of the milling balls as well as the milling time were systematically varied. The prepared powders were characterized by X-ray diffraction, particle size analysis, and scanning electron microscopy. Thermal characteristics were studied using a custom wire-ignition setup and differential scanning calorimetry. Results show that the uniformity of mixing and reactivity of the nanocomposite powders can be improved using milling at cryogenic temperatures.