Submillimeter‐wavelength, linear‐absorption spectroscopy has been applied as a chemical diagnostic of a reactive‐ion etching plasma in a modified capacitively coupled Gaseous Electronics Conference (GEC) reactor. Approximately 1 mW of narrow‐band (< 10 kHz) submillimeter radiation between 450 GHz and 750 GHz is produced using a backward‐wave oscillator (BWO). The submillimeter method offers high sensitivity for the ≈ 1 MHz linewidth, Doppler‐broadened absorption lines typical of gas‐phase molecules at a total pressure of less than 133 Pa (1 Torr). A large variety of molecules can be detected, limited primarily by the need for a permanent electric dipole moment and for accurate line frequency predictions, which are often available in the literature. The capabilities of the diagnostic method have been demonstrated by the following three applications: 1) the measurement of water‐vapor contamination in the reactor and in the precursor gas; 2) the assessment of progress in the cleaning of the reactor; and 3) the determination of the endpoint in the etching of a SiO2thin film on silicon. © 2003 American Institute of Physics