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Pyrolysis(PY)-GC-MS has been used to characterize the chemical nature of the organic matrix of three surface waters derived from diverse locations inNorth America. The chemical fingerprints of water samples treated by conventional and enhanced coagulation are presented and are statistically analyzed relative to their disinfectant byproduct (DBP) yield. This approach seeks to establish quantitative structure-function relationships between chemical components of the organic matrix and DBP formation potential, and illustrates a number of insightful trends among the pyrolysis fragments of these waters. The aromatic signature of these waters was poorly correlated to trihalomethane formation potential (THMFP). For a set of critical pyrolysis fragments identified for these samples, a general pattern was found for the precursor material of THMFP, whereas the chemical nature of precursors for haloacetic acid (HAA) appeared to be more source specific. Furthermore, the precursors to THM and HAA showed distinct chemical characteristics as revealed by PY-GC-MS. Pyrolysis-GC-MS is an analytical technique which has been used extensively in industry, soil science and geochemistry to study the structure of complex, non-volatile, organic macromolecules and recently has been applied to the study of natural organic material (NOM) occurring in systems of interest to environmental engineers. Pyrolysis (PY) is a method that thermally cleaves an organic molecule into volatile fragments which are then separated by gas chromatography (GC) and identified by mass spectrometry (MS). Under controlled conditions this technique yields a reproducible fragmentation pattern, or fingerprint, which is highly characteristic of the parent organic material.In this research PY-GC-MS has been used to monitor the changes in organic quality produced by the coagulation of surface waters from different locations. The primary goal of this work was to characterize by PY-GC-MS the chemical nature of disinfection byproduct (DBP) precursors in different types of waters treated by conventional and optimized coagulation. Specifically, the objectives of this work were to: 1) Chemically fingerprint by PY-GC-MS the organic matrix of raw and treated waters from 3 locations and describe qualitatively the chemical nature of NOM removed under various conditions of coagulation; 2) Evaluate statistically PY-GC-MS data 3