Corona current is produced by interactions between an inhomogeneous electric field and gas. In this paper, the impact of the field on the gaseous medium is studied by measuring and analyzing voltage-current characteristics in several gas mixtures. As the work requires strict control of the medium, with mixture compositions guaranteed to ∼ 0.1 ppm, the corona is created in a special apparatus that facilitates medium handling. Gas chromatographic and spectroscopic techniques have been used to verify the results. It has been established that several trace concentrations of certain medium additives have a considerable impact on the magnitude of the corona current, and on the electrical dielectric properties and breakdown strength of the medium. Role of the additives in the corona medium is analyzed in terms of their chemical properties and their efficacy in suppressiong the breakdown. The additives, divided into groups according to their Milliken-Jaffe energy, are classified in terms of their impact on the corona current and breakdown.