Commercially available single-walled carbon nanotubes (SWCNTs) contain large percentages of metal and carbonaceous impurities. These fractions influence the SWCNT physical properties and performance, yet their chemical compositions are not well defined. This lack of information also precludes accurate environmental risk assessments for specific SWCNT stocks, which emerging local legislation requires of nanomaterial manufacturers. To address these needs, we measured the elemental, molecular, and stable carbon isotope compositions of commercially available SWCNTs. As expected, catalytic metals occurred at per cent levels (1.3–29%), but purified materials also contained unexpected metals (e.g., Cu, Pb at 0.1–0.3 ppt). Nitrogen contents (up to 0.48%) were typically greater in arc-produced SWCNTs than in those derived from chemical vapor deposition. Toluene-extractable materials contributed less than 5% of the total mass of the SWCNTs. Internal standard losses during dichloromethane extractions suggested that metals are available for reductive dehalogenation reactions, ultimately resulting in the degradation of aromatic internal standards. The carbon isotope content of the extracted material suggested that SWCNTs acquired much of their carbonaceous contamination from their storage environment. Some of the SWCNTs, themselves, were highly depleted in ¹³C relative to petroleum-derived chemicals. The distinct carbon isotopic signatures and unique metal 'fingerprints' may be useful as environmental tracers allowing assessment of SWCNT sources to the environment.