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 from when using water (oxidative degradation experiments), because the voltage drop across the fluids is different. Consequently, for projects involving aqueous reactions systems, operational conditions cannot be met by the standard actinometer. Water exhibits a high absorption cross-section for VUV irradiation (lambda < 190 nm) and homolyzes mainly into hydroxyl radicals and hydrogen atoms. Hydroxyl radicals, but not hydrogen atoms, are very efficiently scavenged by methanol molecules, and under defined conditions, the rate of production of hydroxyl radicals may be determined from the rate of degradation of methanol dissolved in the aqueous reaction system. The parameters affecting primarily the rate of methanol degradation, i.e., the incident photon rate, the concentration of dissolved molecular oxygen, the initial methanol concentration, and the flow parameters in the photochemical reactor, were determined and optimized for a general actinometric procedure. A normalized diagram of the incident photon rate versus the initial methanol concentration allows one to determine the boundary conditions under which the rate of methanol degradation may be used to evaluate the production rate of hydroxyl radicals for Xe-excimer light sources of different radiant power and independent of their geometry. Having determined both the rate of production of hydroxyl radicals and the rate of methanol degradation, and having calibrated the corresponding incident photon rates by the cis-trans isomerization of cyclooctene (standard actinometry) in an experimental setup in which the emitted photon rate does not depend on the reaction medium, the quantum yield of the homolysis of water by VUV irradiation from Xe-excimer Tamps may finally be calculated.