Diamond thin films have been deposited directly on stainless steel substrates at low pressure and low temperature conditions over the range of methane (CH4) to hydrogen (H2) ratios of 4% to 9% using electron-cyclotron-resonance microwave plasma-assisted chemical vapor deposition (PACVD) technique. In situ Fourier transform infrared spectroscopy, employed in a reflection-absorption geometry (FTIRRAS), and optical emission spectroscopy have been used to study the plasma species adsorbed on the substrate surfaces as well as the species above the substrates surfaces both before and during the nucleation and film growth. It is demonstrated that these techniques can provide useful information on the early stages of diamond growth. When correlated with film properties measured by Raman spectroscopy and scanning electron microscopy, the results from FTIRRAS indicate that the absorption of the graphitic and diamond phases are related to the ratio of CH4 to H2 and can be identified at the early stages of film growth.