Plasma electrolytic oxidation (PEO), also called plasma anodizing or micro arc oxidation (MAO), is an appropriate method to improve surface characteristics of valve metals. Micro arc discharges, which appear at the working piece in an aqueous electrolyte under strong anodic polarization, are typical for this process. The discharge initiation is discussed controversially. Contradicting theories, which are mainly based on investigations of tantalum, exist concerning the meaning of electrolytic anions, which are initially incorporated in the anodic film. Dual step polarization and PEO experiments were carried out on aluminum, magnesium, and titanium, which are in practical use as lightweight construction materials. For Al 99.5, AZ31, and TiAl6V4 it could be shown that the incorporation of electrolytic anions in the anodic film is not required for the ignition process. Also, the influence of electrolyte conductivity on the ignition voltage was investigated by polarization experiments with silicate, aluminate, and hydroxide solutions using the example of AZ31. It has been demonstrated that not the electrical electrolyte conductivity, but its specific electron injection ability at the electrolyte/substrate interface defines the ignition voltage. This injection ability depends on the kind and concentration of the electrolytic anions.