The kinetic investigation of photodegradation of the natural organic matter ( NOM) in natural lake water was conducted using two low-pressure mercury lamps of the same nominal power: a) with suprasil quartz envelope emitting at both wavelengths 2254 nm and 185 nm ( hereafter VUV); b) with standard quartz envelope emitting only at 254 nm ( hereafter UV). In comparison to the UV process, the addition of 185 nm irradiation ( VUV process) increased the degradation rate ten-fold overall at lower and seventeen-fold at higher Reynolds numbers, respectively. The hydrodynamic characteristics of the photoreactor are of crucial importance for total energy efficiency for VUV processes.