A new ion chromatography electrospray tandem mass spectrometry (IC-ESI/MS/MS) method has been developed for quantification and confirmation of chlorate (ClO3-) in environmental samples. The method involves the electro-chemical generation of isotopically labeled chlorate internal standard (Cl18O3-) using 18O water (H218O). The standard was added to all samples prior to analysis thereby minimizing the matrix effects that are associated with common ions without the need for expensive sample pretreatments. The method detection limit (MDL) for ClO3- was 2 ng L-1 for a 1 mL volume sample injection. The proposed method was successfully applied to analyze ClO3- in difficult environmental samples including soil and plant leachates. The IC-ESI/MS/MS method described here was also compared to established EPA method 317.0 for ClO3- analysis. Samples collected from a variety of environments previously shown to contain natural perchlorate (ClO 4-) occurrence were analyzed using the proposed method and ClO3- was found to co-occur with ClO4 - at concentrations ranging from <2 ng L-1 in precipitation from Texas and Puerto Rico to >500 mg kg-1 in caliche salt deposits from the Atacama Desert in Chile. Relatively low concentrations of ClO3- in some natural groundwater samples (<0.1 μg L-1) analyzed in this work may indicate lower stability when compared to ClO4- in the subsurface. The high concentrations of ClO3- in caliches and soils (3-6 orders of magnitude greater) as compared to precipitation samples indicate that ClO3-, like ClO4-, may be atmospherically produced and deposited, then concentrated in dry soils, and is possibly a minor component in the biogeochemical cycle of chlorine.