Utilities are required to measure TOC concentrations in treatment plant influents and waters that contain a variety of dissolved molecules and organic particles. Uncertainties about the analytical accuracy of TOC measurements are the result of inadequate proficiency evaluation standards. The three primary research goals of the project were to: (1) develop total organic carbon (TOC) standards that can be used with all TOC methodologies to evaluate performance; (2) determine the ability of different TOC methodologies to accurately measure TOC; (3) test the ruggedness of the proficiency standards in a round-robin survey involving a large number of laboratories. A fourth goal was to determine the contribution of natural organic particles and suspended organic carbon (SOC) in the formation of DBPs. In general, all laboratories performed well in the analysis of an easily oxidized standard, potassium hydrogen phthalate (KHP). In contrast, several analysts had difficulty in efficiently separating an inorganic C (IC) amendment from the KHP standard when a 30 mg IC/L spike was added to the standard. Averaged over all instruments and analysts, standards that were solutions were analyzed with 92 9 per cent recovery, while standards that were particles were analyzed with 55 21 per cent recovery. The project data suggest that particles can compete with dissolved precursor material in reactions with applied disinfectants and contribute DBP precursors Originally published by AwwaRF for its subscribers in 2003