Stable carbon isotope (δ(13C)) analysis can provide information concerning the starting materials and the production process of a material. Carbon nanotubes (CNTs) are produced using a variety of starting materials, catalysts, and production methods. The use of δ(13C) as a tool to infer the nature of starting materials to gain insight into the mechanics of CNT growth was evaluated. The production process of NRC's SWCNT-1 was traced via the δ(13C) measurement of the available starting materials, intermediate products, and the final product. As isotopic fractionation is likely negligible at high temperatures, the δ(13C) value of the starting materials was reflected in the δ(13C) value of the final CNT product. For commercially available CNTs, the estimated δ(13C) values of identified starting materials were related to the δ(13C) signatures of CNTs. Using this information and the δ(13C) values of CNTs, the nature of unknown carbon sources was inferred for some samples. The use of δ(13C) analysis may be used as a tracer to differentiate between those processes that use relatively 13C-depleted carbon source(s) such as carbon monoxide, methane, or natural gas, and those that do not.