Adsorption of natural organic matter (NOM) by granular activated carbon (GAC) was studied. Three different carbons of different origin were initially used. The physical properties were studied by means of N-2 adsorption. Chemical properties were studied by means of thermogravimetric analysis (TGA), acid-base titrations, and FTIR. Only one of the carbons showed a broad adsorption band in the 1300-1000 cm(-1) region in FTIR spectra, which can be assigned to C-O stretching and O-H bending modes of alcoholic, phenolic, and carboxylic groups. Adsorption of NOM was studied by batch adsorption experiments. Uptake of NOM by the different carbons was evaluated by UV absorbance, disinfection byproduct formation potential tests, and HPLC-SEC chromatography. Freundlich equation was used to fit equilibrium data. pH(PZC) and overall surface basicity were shown to improve the removal of THM precursors. Differences in the molecular weight distribution of the adsorbed material by different carbons were reported. A clear con-elation was found between the reduction in the THM formation capacity of sample and the reduction in intensity of a specific peak in SEC chromatograms. Furthermore, THMFP tests showed the existence of some fractions of NOM not adsorbable with activated carbons and undetected by measurement of DOC.