The goal of this study was to determine the effect of UV irradiation with a polychromatic spectrum on natural organic matter. Several irradiation tests were carried out with or without cut-off of wavelengths lower than 240 nm on water samples coming from different drinking water plants. DOC, BDOC, SEC analyses, chlorine demand, nitrate and nitrite concentration measurements were made. Changes were noticed as regarding SEC and chlorine demand analyses. Indeed a consistent trend of breaking-up NOM molecules into smaller fragments was observed. Moreover, the chlorine demand increased with the dose when the cut-off filter was not applied, whereas a maximum value resulted when the cut-off filter was applied. Most of these results were obtained at high UV doses (40000 J x m(-2)), suggesting that UV irradiation would not have a noticeable effect on the water samples tested at doses usually used for drinking water treatment.