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Perchlorate (ClO4-) is ubiquitous in the environment It is produced naturally by atmospheric photochemical reactions, and also is synthesized in large quantities for military, aerospace, and industrial applications. Nitrate-enriched salt deposits of the Atacama Desert (Chile) contain high concentrations of natural ClO4-, and have been exported worldwide since the mid-1800s for use in agriculture. The widespread introduction of synthetic and agricultural ClO4- into the environment has contaminated numerous municipal water supplies. Stable isotope ratio measurements of Cl and O have been applied for discrimination of different ClO4- sources in the environment. This study explores the potential of Cl-36 measurements for further improving the discrimination Of ClO4- sources. Groundwater and desert soil samples from the southwestern United States (U.S.) contain ClO4- having high Cl-36 abundances (Cl-36/Cl = 3100 x 10(-15) to 28,800 x 10(-15)), compared with those from the Atacama Desert (Cl-36/Cl = 0.9 x 10(-15) to 590 x 10(-15)) and synthetic ClO4- reagents and products (Cl-36/Cl = 0.0 x 10(-15) to 40 x 10(-15)). In conjunction with stable Cl and O isotope ratios, Cl-36 data provide a clear distinction among three principal ClO4- source types in the environment of the southwestern U.S.