Plasma created during EDM is systematically investigated with optical emission spectroscopy. Typical spectra show a strong H,,, and continuum radiation, with many lines emitted by impurities coming from electrode and workpiece materials. The dielectric molecules are cracked by the discharge. Time-resolved spectroscopy shows that the electron density is extreme at the beginning of the discharge. This causes merging of lines, strong Stark broadening and shift of the H,, line. The electron temperature is low and remains roughly constant around 0.7 eV The low temperature, the high density and other spectroscopic evidences prove that the EDM plasma is non-ideal. (C) 2004 Elsevier B.V. All rights reserved.