The mass spectra of positive ions and negative ions from CF4 and CF4/O2 rf discharges with and without an etching silicon wafer are presented. The positive ion spectra are largely consistent with those obtained by prior investigations and do not exhibit large changes with the addition of silicon to the discharge. Only one silicon containing positive ion was found, SiF3+ at 85 amu. The negative ion spectra changes considerably with the addition of silicon to the discharge indicating that many of the silicon etching products tend to form negative ions rather than positive ions (or neutral species). In particular, the signal from SiF5− at 123 amu is very large, even larger than the signal of F− at 19 amu when silicon is present. (SiOF3−, SiOF4−, and Si2O2F5− can also have large signal intensities.) While atomic and molecular fluorine are difficult to detect in the positive ion spectra, they are both easily found as negative ions. Some of the relationships between the negative ion signals are consistent with the reactions proposed by C. J. Mogab, A. C. Adams, and D. L. Flamm, J. Appl. Phys. 49, 3796 (1978) and G. Smolinsky and D. L. Flamm, J. Appl. Phys. 50, 4982 (1979).