Calculations of the ion composition of plasma in the Ar-CO and Ar-CO-O₂ mixtures are performed under conditions typical active medium of an electric ionization CO-laser. It is shown that, in spite of great uncertainty as the regards the rate constants of a number of ion-molecule reactions, one can state that the (CO)_n⁺ (n = 2, 3) ions dominate in the Ar-CO mixture. The total concentration of these ions, the concentration of electrons, and the time behavior of these quantities may be calculated with sufficient accuracy for the times t> 10^-8-10^-6 s, depending on the experimental conditions. The injection of O₂ in the Ar-CO mixture may result in a noticeable increase of the concentration of charged particles in a discharge and, consequently, in an increase of energy input in the active medium. The mechanism of influence of oxygen on the concentration of charged particles is based on the replacement of the dominating (CO)_n⁺ ion by O₂⁺.