The recently developed technique of real time spectroscopic ellipsometry (SE) has been applied to characterize the nucleation of diamond onc‐Si by W filament‐assisted chemical vapor deposition, leading to improved control over the process. Specifically, techniques are developed which minimize W contamination at the diamond/substrate interface; calibrations are performed which determine the temperature of the top ∼250 A˚ of the substrate under growth conditions; and alterations in gas flow conditions are implemented in response to diamond growth for a reduced induction time. With these procedures in place, real time SE provides the induction time, nucleation density, and mass thickness, and is in quantitative agreement withexsituscanning electron microscopy.