Dissolved organic matter (DOM) in two surface waters (one with low specific ultraviolet absorbance (SUVA) and the other with high SUVA) were fractionated using fivephysicochemical separation processes (activated carbon, XAD-8 batch adsorption, alum coagulation, XAD-8 column fractionation and ultrafiltration). Over forty DOM fractions were obtained for each water. The fractions were characterized using SUVA. Trihalomethane (THMs) and haloacetic acid (HAA9) formation after chlorination of all fractions was quantified. For each natural water, a single and strong correlation was observed between the SUVA values of DOM fractions and their THM and HAA9 formations, independent of the separation processes used to obtain the fractions. Therefore, the correlation obtained for each water appears to represent its natural disinfection byproduct (DBP) reactivity profile. However, SUVA is not a universal predictor of DOM reactivity because a unique DBP reactivity profile was obtained for each water tested. DBP speciation also correlated well with SUVA of DOM fractions in a single water. High-SUVA DOM fractions produced more THMs whereas low-SUVA fractions were more reactive in forming HAA9. Formation of trichloroacetic acid was dominant over dichloroacetic acid for high-SUVA fractions, whereas the opposite was observed for low-SUVA fractions. DBP reactivity profile provides a promising approach for on-line monitoring and control of DBP formation in practical applications. Understanding how reactivity is correlated to SUVA may allow utilities to optimize the degree of treatment required to comply with D/DBP regulations. Includes 16 references, table, figures.