A sensitive method for determining total organic carbon at microgram per liter levels in industrial, environmental, and drinking waters by thermal combustion ion chromatography was developed using tube furnace and readily accessable HPLC equipment. To achieve complete oxidation, persulfate (0.25%) was added to oxidize nonvolatile organic compounds in solution and cupric oxide heated at 900 °C to convert volatile organic compounds to CO(2), which was scrubbed in a 20 mL solution of 50 mM KOH with 10 drops of butanol added. The carbonate anion obtained was determined by nonsuppressed ion chromatography using 0.6 mM potassium hydrogen phthalate (KHP) as the eluent. Both surfactants and volatile and nonvolatile organic compounds commonly found in environmental waters give highly repeatable recoveries close to 100%. The detection limit (S/N = 2) and linear range for a 1 L water sample are 2 μg of C L(-)(1) and 10-2500 μg of C L(-)(1), respectively, and they can be adjusted using samples ranging from 100 mL to 2 L. Good repeatablity (RSD less than 10%) and close to 100% recoveries were obtained for KHP added to real samples such as deionized, mineral, tap, and river water and seawater. Compared with the ASTM D2579 method, the method developed is 3 orders of magnitude more sensitive, more accurate, and reliable in determining samples with low total organic carbon values and more flexible in adjusting the linear range and sensitivity using variable sample sizes.