Basic chemical processes involved in etching of silicon nitride with a CF4/O2radio frequency plasma have been investigated using optical emission spectroscopy. Spectra have been recorded in the range 2400–7050 Å and comparison has been made of the intensity of more than 60 atomic and molecular lines in the presence and absence of silicon nitride coated wafers. We identify a number of species as important etching agents including metastable F(4P), CF2, and metastable F+(1D,1S). Our results are consistent with the principle of spin conservation in surface reactions set out in earlier work on Si(100). Our conclusions extend and broadly confirm the results of a previous mass spectrometric study of the same system.