Ultraviolet‐adsorption spectroscopy has been used to examine how the chamber wall affects the concentration of gas‐phase reactants in high‐density etching plasmas. This technique was employed to detect CF2in an inductively coupled discharge used for the selective etching of silicon dioxide relative to silicon nitride and polycrystalline silicon (polysilicon) films. In plasmas containing C2F6and CF4, the concentration of CF2depends strongly on the applied power and operating pressure as well as the amount of polymer on the walls of the chamber. Changes in the conditioning of the chamber during the etch process cause significant variations in the concentration of CF2in the discharge. The selectivity of etching SiO2relative to Si3N4films closely follows the concentration of CF2under a variety of plasma operating conditions. The ability to measure a fundamental plasma characteristic that reflects the level of conditioning of the chamber is an important step in the real‐time monitoring of a reactor parameter that currently can only be determined from postprocess measurements. ©1995 American Institute of Physics.