This paper presents findings from a study of nanocrystalline diamond (NCD) growth in a microwave plasma chemical vapor deposition reactor. NCD films were grown using Ar/H2/CH4 and He/H2/CH4 gas compositions. The resulting films were characterized using Raman spectroscopy, scanning electron microscopy, and atomic force microscopy. Analysis revealed an estimated grain size of the order of 50 nm, growth rates in the range 0.01-0.3 μm/h, and sp3- and sp2-bonded carbon content consistent with that expected for NCD. The C2 Swan band (d3IIg harr; a3IIu) was probed using cavity ring-down spectroscopy to measure the absolute C2(a) number density in the plasma during diamond film growth. The number density in the Ar/H2/CH4 plasmas was in the range from 2 to 4 × 1012 cm-3, but found to be present in quantities too low to measure in the He/H2/CH4 plasmas. Optical emission spectrometry was employed to determine the relative densities of the C 2 excited state (d) in the plasma. The fact that similar NCD material was grown whether using Ar or He as the carrier gas suggests that C2 does not play a major role in the growth of nanocrystalline diamond.