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Chloride is used as conservative tracers to estimate hydrological pathways. Field data on treatment of municipal solid waste (MSW) leachate show that Cl is reduced in aerobic treatment. Here laboratory experiments simulating MSW leachate treatment are conducted to see if electrical conductivity (EC) reduction can be duplicated in the laboratory, and to find probable chemical or physical factors explaining EC and chloride reduction. The experiments include three leachate, eight artificial, and three control samples. Mean reduction of EC in the respirometers was 17% for the leachate and 25% in the artificial samples. There was close to zero reduction in the controls. Mean reduction of chloride was 45% in the artificial samples and 9% in the controls. The oxygen consumption was lower in the leachate samples during aeration, probably due to inhibitory compounds. There was no reduction in EC after chemical coagulation with A1 and organic coagulants, 31% reduction in the aerobic system, and 46% in the anaerobic system. Total organic carbon significantly influenced the oxygen consumption. The reduction of chloride and EC depended on the acidity (pH) of the leachate. Centrifugation showed an effect on EC and chloride concentration. Based on these results it seems that the reduction of chloride in leachate treatment depends on the formation of organic precipitates. DOI: 10.1061/(ASCE)0733-9372(2007)133:6(659) CE Database subject headings: Conductivity; Chlorides; Leaching; Municipal wastes; Aerobic treatment.