Instances of counterfeit refrigerants causing violent and unexpected explosions@ resulting in multiple fatalities@ have been reported in mobile refrigeration units around the world. In addition@ counterfeit refrigerants have caused system reliability issues in numerous air-conditioning applications. It was initially believed the inclusion of methyl chloride (R-40) in the refrigerant composition caused these explosions and reliability issues. ASHRAE Research Project RP-1665 was commissioned to examine reactivity of R-40 with R-134a (methylene chloride) refrigeration system materials. R-40 reactivity@ in concentration ranges of 0.01% to 10%@ was studied in the presence of R-134a@ polyolester (POE) lubricant@ aluminum 1100 metal@ aluminum 380 metal@ iron metal@ copper metal@ sodium potassiumaluminumsilicate zeolite@and alumina catalysts. R-40 was shown to have varying levels of reactivity??generally mild@ but showing the potential for catastrophic reactivity. This paper contains a summary of the work from ASHRAE RP-1665 and provides insights into the impact of R-40 contamination by providing the chemistry of the reactions@ preventative safeguards@ threshold levels@ and assessment procedures.