Several thermite powders with nominal composition 2Al3CuO were prepared by arrested reactive milling using different milling times. Their structure, morphology, and compositions were characterized using scanning electron microscopy and energy dispersive X-ray spectroscopy. The powder particles comprise a fully dense Al matrix with CuO inclusions. The dimensions of the CuO inclusions are around 100nm and are effectively the same for all prepared powders. The number of inclusions per unit of mass of the composite particles increases with increased milling times, while unattached CuO particles are present in the samples prepared with shorter milling times. The difference in the sample compositions results in different heat flow rates measured in differential scanning calorimetry (DSC) and constant temperature micro-calorimetry experiments. The DSC data collected at varied heating rates and isothermal calorimetry measurements at different temperatures are interpreted using a multistep reaction model proposed earlier and describing kinetics of redox reaction in the Al/CuO thermites. The model remains valid for composite powders prepared with different milling conditions and, therefore, characterized by different aluminum/copper oxide ratios. To accurately describe the rate of redox reactions in specific nanocomposite powders, the model must account for the size of CuO inclusions and the number of inclusions per unit of mass in the nanocomposite particle.