A high‐pressure (2×4×77) cm3UV‐sustained discharge was operated in a (1:3:9, CO:N2:He) gas mixture doped with either NO or C2H4. The electrical energy loading into the gas was parameterized with respect to the UV source energy, additive density, reduced temperature, and sustainer voltage. All the results were consistent with a discharge operated below the glow voltage and sustained entirely by photoionization. A maximum energy loading of about 400 J/liter was achieved.