The effects of diluting ultrahigh-frequency CxFy plasmas with Xe, Ar, and He are studied by optical emission spectroscopy and Langmuir probe measurement. The Ar and He dilutions are found to shift the electron energy distribution function toward higher energies. Ar and Xe also drastically increase the electron density, whereas He does not change the electron density. As a result, Ar dilution increases the ratio of ion density to CF2 density. It is expected that the Ar dilution changes the balance between the etching and the polymer deposition and significantly affects the SiO2 etching characteristics. He dilution causes a high degree of dissociation and a large increase in the density of F atoms for CxFy gases, in addition to the dilution of CF2 radical density, because of its high threshold energy for ionization. The Xe dilution drastically increases the n(e), while it maintains a lower electron temperature.