This study compared the differential absorbance spectroscopy of the raw water oxidized with chlorine and chlorine dioxide to explore the reactions between oxidants and natural organic matter in disinfection by-products (DBPs) formation. The correlations between the differential absorbance at 272 nm (ΔA272) and 254 nm (ΔA254) and DBPs formation in chlorinated water were also examined, respectively. The intensity of the differential absorbance increased with increasing doses for both chlorine and chlorine dioxide. The sites in natural organic matter oxidized with chlorine and chlorine dioxidate are different. Unlike the well defined band at 272 nm and 254 nm in differential spectra of water oxidized with chlorine, a well defined band at 220 nm was found in water oxidized with chlorine dioxide. The behavior of -ΔA272 during chlorination process was determined to be strongly correlated with DBPs concentrations quantified by linear equations with R2 values > 0.95. A weak correlation (R2 value 254, indicating the formation of DBPs independent of destruction in UV-absorbing sites.