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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.