Equal amounts of carbon-12 methyl halides and carbon-13 methane, along with hydrogen, have been introduced into a hot-filament diamond CVD chamber. The isotopic ratios of the as-deposited diamond films on tantalum have been measured by Raman spectroscopy. It was found that methyl chloride yielded carbon-12 enriched diamond films, while the other methyl halides resulted in equal amounts of carbon-13 and carbon-12 in the films. Furthermore, the carbon-12 enrichment was more enhanced as the substrate temperature was lowered. Matrix-isolation FTIR analyses of the gas samples collected during the deposition indicated that there was no straightforward agreement between the 12C/13C ratios of the gas phase species, methane and acetylene, and that of the diamond films. The results imply the presence of a new chlorocarbon radical such as CH2Cl, which is postulated as a more effective growth species than the methyl radical.