Real time spectroellipsometry has been applied to determine the deposition rate and thickness evolution of the nondiamond (sp2‐bonded) carbon volume fraction in very thin (<1000 A˚), but fully coalesced, nanocrystalline diamond films prepared on Si substrates by microwave plasma‐enhanced chemical vapor deposition from gas mixtures of CO and H2. At a substrate temperature of ∼800 °C, high quality diamond films can be obtained over two orders of magnitude in the CO/H2gas flow ratio, from 0.04, the lowest value explored, to ∼5. A well‐defined minimum in thesp2C volume fraction (0.03 in a 600 A˚ film) is observed for a CO/H2ratio of 0.2, corresponding to the C–H–O diamond‐growth phase‐diagram coordinateXH/&Sgr;=[H]/{[H]+[C]} of 0.9. Under these conditions, the deposition rate increases with increasing temperature over the range of ∼400–800 °C with an activation energy of 8 kcal/mol, behavior identical to that observed for diamond film growth from a CH4/H2ratio of 0.01. This observation shows that the dominant film precursors in the diamond growth process from CO/H2=0.2 are hydrocarbons whose flux at the growing film surface is controlled through the reaction of excited CO with H or H2in the plasma. A broad subsidiary minimum in thesp2C content is observed, centered near a CO/H2ratio of 2, corresponding to anXH/&Sgr;value of ∼0.5. Under these gas flow conditions, the deposition rate is a complicated function of temperature, exhibiting a peak near 550 °C. This peak shifts to lower temperature with further increases in the CO/H2ratio above 2, suggesting a nonhydrocarbon precursor and a different growth mechanism for diamond prepared at high CO/H2ratio and low temperature. ©1996 American Institute of Physics.