This study examines the unique reaction behaviors associated with polytetrafluoroethylene (PTFE) and nanometer scale aluminum (Al) particles compared with micron-scale Al particles. DSC and TG analyses were performed in an argon environment on both nanometer and micron scale mixtures revealing lower onset temperatures and larger exothermic activity for the nanometer scale Al mixture. The increased sensitivity and exothermicity is caused by a pre-ignition reaction (PIR) unique to the nano-Al mixture. Experiments show the reaction to be the fluorination of the Al particles' passivation shells. The decrease in alumina content caused by larger Al particles lowers the exothermic effect of the PIR. 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 Al particle size effects on the Al/PTFE reaction.