Real-time spectroscopic ellipsometry (RTSE) has been applied to study the growth of nanocrystalline diamond thin films by microwave plasma-enhanced chemical vapor deposition on silicon substrates. The goal of this research is to characterize the diamond film growth process as a function of the gas source and substrate temperature, comparing the results obtained using various mixtures of CO and H-2 With those Obtained from a standard mixture of CH4 highly diluted in H-2. The capabilities of RTSE have been exploited to establish the true near-surface substrate temperature under the specific diamond film growth conditions, as well as the deposition rates for a succession of films prepared on the same substrate under different conditions. The latter capability allows large regions of parameter space to be scanned expeditiously. As a result of this study, a low-temperature growth process has been identified that yields high deposition rates (up to 2.5 mu m/h) at relatively low microwave plasma powers (0.5 kW). In contrast to the commonly-used H-2-rich mixtures of CH4 or CO and H-2 that exhibit monotonic reductions in the growth rate with decreasing substrate temperature from 800 to 400 degrees C, CO-rich mixtures of CO and H-2 exhibit an increase and a well-defined maximum as the temperature is reduced over this range. At a CO/H-2 gas flow ratio of 18, for example, the growth rate peaks near 450 degrees C and is a factor of similar to 20 higher than that obtained with the standard H-2-rich mixtures of CH4/H-2 and CO/H-2. These observations suggest a different diamond growth mechanism from the CO-rich mixtures of CO/H-2 with potentially important applications for low-temperature substrate materials.