High density plasmas generated using gas mixtures of methane, hydrogen, and argon are characterized using mass spectrometry, optical emission spectroscopy, and three dimensional Langmuir probing. Such plasmas are commonly used to etch compound semiconductors. In this work we examine the chemical and electrical properties of the flux to the region where substrates art: placed during processing. The dominant species in the flux are identified as II, H+, CH3, CH3+, Ar, and ArH+. Plasma parameters in the source region include electron temperatures of 4-9 eV, plasma densities of 1-5 x 10(11) cm(-3), and plasma potentials of 24-44 V as process conditions art: varied. These parameters are considerably reduced in the process region of the plasma to electron temperatures of 2-6 eV, plasma densities of 1 x 10(9) to 2.5 x 10(10) cm(-3), and plasma potentials of 3-14 V. Mass and optical emission spectral data are correlated to Langmuir probe results and the effects of varying process conditions on plasma properties are presented and discussed.