Chlorine dioxide is widely used as a pre-oxidant and disinfectant but it has the disadvantage of inducing the formation of the inorganic by-products chlorites and chlorates. It is therefore of interest to decrease the chlorine dioxide demand and oxidation, and/or removal of natural organic matter (NOM). The objective of this study was to estimate the efficiency of the adsorption of oxidised and unoxidised NOM on granular activated carbon (GAC) filters. For this purpose, three pairs of columns filled with three types of GAC were set. The study showed that the efficiency of adsorption increased after the pre-oxidation of NOM with ClO2. GAC filters also efficiently removed the unwanted inorganic by-products.