Plasma created during electrical discharge machining is systematically investigated using optical emission spectroscopy. Typical spectra show a strong H-alpha and continuum radiation, with many lines emitted by impurities coming from electrode and workpiece materials. The dielectric molecules are cracked by the discharge. Changing polarity affects the electrode wear and workpiece erosion rates, which can be qualitatively seen on the spectra. Time-resolved spectroscopy shows that the plasma density reaches 2 x 10(18) cm(-3) at the beginning of the discharge. This extreme density causes the merging of lines, strong Stark broadening and shift of the H-alpha line. Afterwards, the density decreases rapidly with time. The electron temperature remains roughly constant around 0.7 eV. The low temperature and the high density measured prove that the EDM plasma is non-ideal (Gamma similar or equal to 0.45). Absence of the H-beta line, asymmetric shape of the H-alpha line and complex structures around H-alpha are other spectroscopic evidences of the plasma non-ideality.