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BaSO4 precipitated from mixed salt solutions by common techniques for SO42- isotopic analysis may contain quantities of H2O and NO3- that introduce errors in 0 isotope measurements. Experiments with synthetic solutions indicate that delta O-18 values of CO produced by decomposition of precipitated BaSO4 in a carbon reactor may be either too low or too high, depending on the relative concentrations of SO42- and NO3- and the delta O-18 values of the H2O, NO3-, and SO42-. Typical delta O-18 errors are of the order of 0.5 to 1 parts per thousand. in many sample types, and can be larger in samples containing atmospheric NO3-, which can cause similar errors in delta O-17 and Delta O-17. These errors can be reduced by (1) ion chromatographic separation of SO42- from NO3-, (2) increasing the salinity of the solutions before precipitating BaSO4 to minimize incorporation of H2O, (3) heating BaSO4 under vacuum to remove H2O, (4) preparing isotopic reference materials as aqueous samples to mimic the conditions of the samples, and (5) adjusting measured delta O-18 values based on amounts and isotopic compositions of coexisting H2O and NO3-. These procedures are demonstrated for SO42- isotopic reference materials, synthetic solutions with isotopically known reagents, atmospheric deposition from Shenandoah National Park, Virginia, USA, and sulfate salt deposits from the Atacama Desert, Chile, and Mojave Desert, California, USA. These results have implications for the calibration and use of 0 isotope data in studies of SO42- sources and reaction mechanisms. Published in 2008 by John Wiley & Sons, Ltd.