Reactive mixtures of aluminum (A1) and polytetrafluoroethylene (PTFE or Teflon) have applications in propellants, explosives, and pyrotechnics. This study examines the thermal degradation behavior of Teflon and nanometer scale A1 particles compared with micron-scale A1 particles. Differential scanning calorimetry and thermo-gravimetric analyses were performed in an argon environment on both nanometer and micron scale particulate mixtures revealing lower ignition temperatures and larger exothermic activity for the nanometer A1/Teflon mixture. This increased ignition sensitivity and exothermicity is caused by a pre-ignition reaction unique to the nano-A1 mixture. Experiments suggest that the pre-ignition reaction mechanism is controlled by the fluorination of the A1 particle passivation shell. Micron-scale A1 particles have a lower specific surface area and therefore the influence of the passivation shell promoting a pre-ignition reaction is reduced. Chemical kinetics are discussed along with particle morphology to explain the thermal degradation process of the mixtures. These results are helpful in the fundamental understanding of A1 particle size effects on the reactivity of A1/Telfon composites.