The ion, electron and atom number densities, and electron temperatures of a cesium plasma generated in a diode have been measured by spectroscopic techniques. The theoretical bases for these techniques are outlined. The errors involved in applying these theories to heavy atom collision‐dominated plasmas are discussed. Working equations for the electron, ion and atom temperatures, and number densities are developed. Among the working equations developed and applied are those for the determination of the electron number density from the continuum intensity and Stark broadening. Temperatures in the cesium plasma were 0.7 ev with ion and electron number densities of the order of 1015cm−3. The agreement of cesium pressure computed from the spectroscopically determined temperatures and number densities and that from vapor pressure curves strongly suggest the existence of thermal equilibrium. The agreement in the values of electron number densities determined from continuum intensity and Stark broadening measurements justify the approximations made in applying these theories to the cesium plasma.