Abstract Etching of polyimide films in CF 4 O 2 plasma discharges is investigated. Etch rates, temperature at the film surface, optical emission intensities from the gas phase, and atomic concentrations on the polymer surface obtained by X-ray photoelectron spectroscopy (XPS) have been measured as a function of gas composition for very small reactor loads. Etch rates and XPS data for small loads are reported for the first time. With minimal contribution from ion bombardment and the approximation of certain reactive radical concentrations in the gas phase and the polymer surface, it has been possible to interpret the measured rates kinetically. A rate expression proposed earlier closely correlates with the measured etch rate data, leading to the conclusion that fluorine atom addition to the aromatic carbons in the polymer, which is known to be critical in determining the polymer radical concentration, is not the rate controlling step.