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Background and Aim: Natural organic matter (NOMs), measured on the basis of organic carbon, produces disinfection by-products precursors (DBPs) during the chlorination process. Some DBPs are carcinogenic. NOMs are not completely removed by conventional water treatment. As a result, in addition to forming DBPs, they support bacterial regrowth in the water distribution systems and cause unpleasant odor and taste and other problems. The objective of this study was to determine organic carbon concentration and DBPs formation potential in drinking water of the water distribution system in Tehran, Iran. Methods and Materials: Water sampling was done by standard methods, monthly between August 2006 and Feb 2007, at four points of the drinking water distribution system in Tehran. Quantitative parameters, including pH, EC, UV254 (UV absorbance at a wavelength of 254nm), dissolved organic carbon (DOC), and specific UV absorbance (SUVA) were determined. DOC and UV-254 of the samples were determined using a Total Organic Carbon (TOC) analyzer and a Lambda 25 UV/Vis spectrophotometer, respectively. SUVA (L/mg.m) was calculated as thea ratio of the UV absorbance at 254 nm (1/m) to DOC (mg/ L). Results: The water content of DOC at the four points of the distribution system was less than 0.7 mg /L. The mean DOC concentration was 0.3±0.14mg/L, ranging between 0.12 and 0.687mg/L, with no monthly variation. The mean SUVA was 1.3±0.7. Conclusion: It can be concluded that the Tehran water treatment systems cannot eliminate completely natural organic matter. SUVA values show, however, that the system can eliminate the majority of hydrophobic compounds. Therefore, formation of haloacetic acids is more probable than that of trihalomathanes. [ABSTRACT FROM AUTHOR]