Ammonium perchlorate has been used since the 1940s in the United States as a component of the solid propellant fuel for rockets and missiles. Perchlorate salts, which are strong oxidants, also are used in fireworks, munitions, airbag inflation systems, highway flares, and matches. Past activities of the military and aerospace industries have led to widespread perchlorate contamination of groundwater. 1 Improvement of routine analytical methods used for the measurement of perchlorate in groundwater since 1997 have resulted in widespread detection of perchlorate throughout the United States in areas where perchlorate salts have been manufactured, stored, or used . 2-4 Lake Mead and the downstream Colorado River contain measurable levels of perchlorate1. Perchlorate has been detected in commercial food products, including vegetables and milk. 5,6 Although the total scope of perchlorate contamination in the United States remains unclear, recent estimates indicate that the drinking water supplies of 15 million people are affected by the contamination of the Colorado River alone. 7 The potentially adverse human health effect of perchlorate ingestion makes it a major public health concern. Perchlorate inhibits iodide uptake by the thyroid causing disruption of normal thyroid function, which can lead to a number of serious health problems, especially pertaining to early neurological development. 8,9 In March 2004, California established a Public Health Goal of 6 μg/L for perchlorate concentration in drinking water, and other states have independently adopted advisory levels ranging from 1 to 18 μg/L. A special committee of the National Academies of Science (NAS) recently reviewed the toxicological data concerning perchlorate, and released a controversial report10 that recommended a safe daily perchlorate dose more than 20 times higher than that endorsed earlier by the U.S. Environmental Protection Agency (USEPA). 11 The USEPA subsequently accepted the NAS recommendation and raised their reference dose from 0.03 to 0.7 μg perchlorate/kg body wt/day. The perchlorate anion (ClO4-) consists of a central chlorine atom in tetrahedral coordination with four oxygen atoms. Perchlorate salts are soluble in water and some organic solvents. When dissolved in water, perchlorate is nonvolatile, stable and kinetically inert with respect to abiotic reduction and oxygen exchange. For example, the exchange of 180-labeled water with HC104 at a concentration of 9 mol/L was insignificant after 63 days at 100 °C, and the half-life for exchange at room temperature is estimated at greater than 100 years. 12 Perchlorate does not adsorb strongly to activated carbon and most mineral surfaces, and it is sufficiently unreactive that it cannot be effectively removed from water by conventional water treatment methods. 13 Perchlorate can be reduced in aqueous solution by some transition metal ions, such as Ti(III), Re(V), V(III), V(II), Mo(III), and RU(II). l3 A variety of different microorganisms have been isolated that can reduce perchlorate under anoxic conditions. 14 These organisms are currently being utilized in several full-scale bioreactor systems to remove perchlorate from groundwater at flow rates as high as 5,000 gallons per minute. 15,16 The presence of persistent plumes of perchlorate in groundwater aquifers indicates that biotic and abiotic reduction of perchlorate under typical oxic groundwater conditions does not lead to significant attenuation. However, the addition of a variety of different substrates is being used to stimulate perchlorate biodegradation in subsurface environments where the anion is otherwise long-lived. 16 The perchlorate used for military and aerospace applications is synthesized by electrolytic oxidation of aqueous chloride brine. Although much of the known perchlorate contamination in the U.S. can be related to such industrial sources, there are some natural sources of perchlorate as well. The origin and abundance of perchlorate from natural sources is poorly known. The best known natural source of perchlorate is within the nitrate-bearing salt deposits of the Atacama Desert (Chile) that have been mined extensively for use as a nitrate source in agricultural ferti1izers. l7 Perchlorate of apparent natural origin also has been detected in a number of ancient evaporite salt deposits,18 and in modern rain and snow samples. 19 At several locations such as in the West Texas panhandle area, perchlorate of possible natural origin has been detected in ground water. 20 The presence of natural sources of perchlorate in groundwater, whether through application of imported perchlorate-bearing fertilizers or from other more local but undefined sources, complicates the issue of liability for groundwater remediation near industrial sources of perchlorate. A practical forensic tool is thus needed that would enable the identification of the source(s) of perchlorate in contaminated aquifers and waterways. Successful applications of stable isotope ratio measurements of N and 0 in studies of nitrate, and C and C1 isotope ratios in studies of chlorinated solvents, have encouraged us to pursue the development of C1 and 0 isotopic analyses for tracing the sources and understanding the behavior of perchlorate in the environment. In this chapter, we review earlier isotopic studies of perchlorate and present new data to show that synthetic and natural perchlorates have distinct chlorine and oxygen isotopic compositions. We also present data for perchlorate extracted from groundwater samples that demonstrate the potential application of stable isotope ratio measurements for environmental forensics.