The ellipsometric film thickness measurement precision for equivalent oxide thickness as prescribed by the International Technology Roadmap for Semiconductors is quite high. Although short‐term precision on a single ellipsometric instrument can be quite high, deviations of measured film thickness from instrument‐to‐instrument and from lab‐to‐lab for short‐term and long‐term periods of time need to be addressed. Since the derived film thickness is dependent on many factors, each one has to be dealt with in turn. These factors include: ellipsometric instrument precision and accuracy, consistency of film/substrate modeling, optical constants, regression analysis, and film surface contamination. Recommendations for standard models and optical constants are given along with the need to ensure high ellipsometric instrument precision and accuracy and controlled film surfaces and environmental conditions. In this study ultra‐thin refers to oxide films starting at 10 nm and being as thin as the native oxide. © 2003 American Institute of Physics