This paper describes work being performed to define natural gas contaminant concentration levels necessary to insure that internal corrosion of compressed natural gas (CNG) cylinders does not constitute a hazard over the lifetimes of the cylinders. Corrosion and stress corrosion cracking (SCC) tests have been performed on typical steel and aluminum CNG cylinder materials using representative natural gases collected from three wells in New York and from an interstate gas transmission pipeline. Also, metallurgical analyses of six used steel CNG cylinders have been performed. No environmentally-induced cracking nor significant corrosion was found in the six cylinders examined. Two steels and an aluminum alloy were not susceptible to SCC in any of the natural gas environments tested. The steels were susceptible to severe SCC in a control environment containing high concentrations of hydrogen sulfide (H2S), but the aluminum alloy was not susceptible in the H2S environment. Additional tests to determine effects of individual contaminant species, metal strength (hardness), and other variables are in progress.