Langmuir probe measurements on , , , and plasmas with concurrent etch rate measurements on and Si have been made in a reactive sputter etching system. At an rf power of 300W and a pressure of 15 mTorr, the ion density is typically , the electron temperature 2 eV, the plasma potential 30 eV, and the ion energy (i.e., the sheath potential) of the order of 500 eV. The dependence of these parameters on rf power, pressure, reactor geometry, and plasma gas composition is investigated, as well as the spatial uniformity of the plasma. For , etch rates vary essentially in proportion to ion densities with all the different plasmas. For Si in plasmas, the anisotropic part of the etch rate also parallels the ion density. From the measured ion densities the ion flux to the substrate is estimated. Thus, etch rates per ion can be determined, which turn out to be similar to those in reactive ion beam etching, although in reactive sputter etching ion energies are much smaller.