Neutral gas density has been measured in both the source and sample regions of an axial electron cyclotron resonance (ECR) plasma device. The measurements involved sampling with a small tube to an external capacitance manometer. The gas density in the central ECR region was found to be strongly dependent on the applied power and to decrease approximately linearly with discharge power. At the maximum applied power of 1250 W, and at an Ar pressure of 0.7 mTorr, the gas density was reduced to 14% of its preplasma value, indicative of significant gas heating and rarefaction. This is equivalent to a gas temperature of 2500 K (0.2 eV), which is consistent with reported Doppler broadening optical emission measurements. At the sample region, approximately 35 cm from the ECR region, the gas density was reduced by gas heating to only 85% of its preplasma value. The electron density was measured in the ECR region by means of a commercial microwave interferometer device. Electron densities, integrated across the width of the chamber, reached ~ 2 × 1012cm -3 at 1250 W and 0.7 mTorr Ar. The ionization fraction at 0.7 mTorr and 1250 W of applied power was 0.31, assuming an average electron density across the chamber. Indications point towards a localization of the plasma in the center of the chamber, which would indicate an ionization fraction of more than 0.50. These results suggest a significant influence of gas temperature and rarefaction on the dynamics of plasma formation and ionization efficiency. In addition, an isotropic, chemical component to many reactive etching systems must be anticipated due to the potentially high gas temperatures in and near the ECR region. © 1990, American Vacuum Society. All rights reserved.