Experimental and computational work was performed to help understand why sub-inerting concentrations of HFC-125 (C2HF5) produced overpressures in the FAA aerosol can explosion test. The fire suppression performance of HCFC-123 (C2HCl2F3) was also investigated to determine whether it may perform better than HFC-125. Thermodynamic analysis shows that both agents increase the overall heat release for lean mixtures containing the aerosol can contents. HFC-125 also increases the overall reaction rate when added to lean mixtures. The overall reaction rate of mixtures containing HCFC-123 is generally lowered when sub-inerting concentrations are added. Experimental results showed that HCFC-123 has a lower minimum inerting concentration (8.9%) than HFC-125 (13.5%). Mixtures containing HCFC-123 were found to produce peak pressures in the 2 L chamber that were estimated to cause overpressures in the FAA chamber. Nitrogen dilution resulting in 20% oxygen in air was successful at eliminating the overpressure of mixtures containing HCFC-123.