With the development of new light sources (Xe-excimer light sources), the vacuum-UV (VUV) photochemistry on a preparative scale is becoming technically feasible. Among the first potential technical applications, VUV photolysis of aqueous systems must be considered as a potential alternative to established “advanced oxidation procedures” (AOP). For the design and dimensioning of corresponding reactors, incident photon rates must be determined. The standard VUV actinometry in condensed phase is the cis−trans isomerization of cyclooctene in n-pentane. The incident photon rate of these new light sources depends on their geometry, the configuration of their electrodes, and the dielectric constant of the solvent in the case where the substrate solution is part of the dielectric barrier; thus actinometric experiments should be made under operational conditions. However, the radiant power density of the excimer sources will be different if n-pentane (standard actinometry) is used as part of the dielectric barrier...