The growth of diamond thin films in enhanced chemical vapor deposition (CVD) processes requires high substrate temperatures (400-1000 degrees C) and gas pressures (1 Torr to 1 atm), as well as high-power excitation of the gas source (e.g., 1 kW microwave plasma). Thus determining the substrate temperature in this severe environment is a challenge. The issue is a critical one since substrate temperature is a key parameter for understanding and optimizing diamond film growth. We have developed and utilized methods for precise (similar to+/-5 degrees C) Si substrate temperature calibration based on rapid-scanning spectroscopic ellipsometry. In this approach the E(1) Critical point energy in Si is used to deduce the temperature of the top similar to 200 Angstrom of the substrate. In addition, the broadening parameter associated with the critical point transitions can provide information on near-surface plasma damage. As an application of the temperature calibration, we have measured the kinetics of diamond film growth by the microwave plasma-enhanced CVD process.