A quadrupolemass spectrometer gives much useful information about plasma etching processes on silicon and its compounds. The present work confirms some of the known or postulated chemistry in the plasma and raises further questions about the understanding of the process. The instrument enables dynamic optimization of process parameters and provides a variety of diagnostic information. In particular, it allows the detection of etching endpoints and thereby provides a sensitive technique for controlling the etching process. The technique appears to substantially reduce or eliminate the need for calibration. The account discusses endpoints for polycrytalline silicon,silicon dioxide, silicon nitride, and some cases involving photoresist.