The light power of a xenon excimer VUV source was determined by means of oxygen actinometiy. It was found that the method is notapplicable for direct measurement of the light power mainly because of the side-reactions of ozone with oxygen atoms. Model calculations led to a simple equation between the quantum yield and the concentrations of ozone and oxygen. The correlation between the overall quantum yield and the ozone and oxygen concentration at room temperature is Φ = 2 - 1.l0[O3]/(O2]1.94, and at atmospheric oxygen pressure Φ = 2 - 520 [O3].