Diamond film deposition studies were carried out in a microwave plasma reactor using feed gases containing methane and acetylene with one reagent labeled with carbon-13. Stable gas-phase species and their carbon-13 content were characterized by downstream mass spectrometry. Raman spectroscopy along with curve-fitting analysis was used to determine the carbon-13 content of diamond and the sp2 carbon impurity Diamond and the sp2 carbon impurity-originated predominantly from methane and/or methyl radical, as opposed to acetylene. Thus, it appears that the differentiation between diamond and sp-2 carbon formation occurs on the growth surface, and not in the gas phase.