The photoionization process for UV‐initiated and UV‐sustained CO laser operation was investigated by measuring the photoion density produced by a single spark source using charge collection. C2H4and NO were studied as potential low‐temperature additives for enhancing photoion production, both of which photoionized by a single‐step process with a conversion efficiency of spark energy into photoionization of about 1%. The photoionization was also found to increase linearly with dissipated spark energy, was independent of the spark circuit parameters, and at constant gas and additive density was also independent of temperature.