The chemistry of diamond chemical vapor deposition has been investigated using carbon- 1 3 labeling of reagents and high gas flow rates which limit potentially complex hydrocarbon chemistry. The sp2 carbon impurity in diamond appears to predominantly originate from methane/methyl radical, as opposed to acetylene. Since methyl radical also appears to be the source of diamond, it is likely that the differentiation between diamond and sp carbon formation occurs on the growth surface, and not in the gas phase. Oxygen species are implicated in hydrogen atom absiraction reactions that increase the concentration of methyl radical and result in higher diamond growth rates. The results are discussed in terms of a mechanism for gas phase chemistry from the combustion literature.