SummaryThis paper deals with theoretical and experimental investigations of Electron free streaming waves in He and Ne dc low pressure discharges. The theoretical description of Electron free streaming waves, based on a ballistic theory, yields through numerical calculations the local amplitude and phase of the wave as a function of the parameters vTC, Z,, vcn and the wave freqnency Co. The amplitudes of Electron free streaming waves, obtained with an improred measuring technique, are in good agreement with the theoretical values. It is shown, t h a t the simultaneous measuring of amplitude and phase of Electron free streaming wave represents a valunble increase in knowledge of diagnostic methods for the internal plasnia piwilmeters vTe and a, (especially at low pressures po 5 Torr).