In a previous study it was observed that reduced sulfur specie (RSS) specifically thiosulfate (S2O32-), competes with perchlorate (ClO4-) for adsorption sites on tailored granular activated carbon (TGAC). This decreases the TGAC ability to removed ClO4- from groundwater effectively. Oxidation is one process that can be used to oxidize these RSS to sulfate, which has been proven to not affect ClO4- adsorption with concentration has high as 250mg/L. The present paper describes research into S2O 32- oxidation by chlorine. Free chlorine was used as a pretreatment method to remove S2O32- from Redlands groundwater in California. Chlorination in the form of hypochlorous acid (HOCl) and hypochlorite ion (OCl-) was used to oxidize S 2O32- to sulfate prior to treatment with tailored granular activated carbon (TGAC). Thiosulfate, RSS, was seen to compete with ClO4- for adsorption sites on the TGAC. Experiments were first conducted synthetically using deionized distilled (DI) water spiked with different concentration of ClO4-, S2O 32- and chlorine depicting a variety of treatment conditions. Conditions include concentrations of 1 and 0.5 mg/L ClO 4- and 1, 0.5 and 0.1 mg/L of S2O 32-. These experiments were carried out as pseudo reactions where free chlorine was in excess. Redlands groundwater was also spiked with S2O32-, the same concentration as background ClO4- concentration that is found in the groundwater. Results showed that DI water spiked with only 1 mg/L ClO 4- broke through at 15,000 bed volumes, while DI water spiked with both 1 mg/L S2O32- and 1 mg/L ClO4- broke through at 8,000 bed volumes, representing a 47% capacity reduction. Moreover, deionized distilled water that was spiked with 1 mg/L ClO4- and just a mere 0.1 mg/L S2O 32- showed breakthrough at 9,400 bed volumes, representing a 37% reduction. Next, the authors also pre-chlorinated DI water with 0.25mg/L of free chlorine as HOCl, while this water also contained 1 mg/L ClO 4- and 0.1 mg/L S2032-; and in this case, ClO4- broke through at 12,000 bed volumes. This indicates that chlorine oxidation of S2O 32- increased the TGAC capacity to remove ClO 4- from 63% to over 80%. As follow-up, Redlands, CA groundwater that contained 30 μg/L ClO4- was spiked with 40 μg/L S2O32-, plus the 0.25 mg/L of HOCl. This RSSCT exhibited no difference for ClO4- breakthrough between the RSSCT that was spiked, 30,000 bed volumes, and the RSSCT that had no S2O32- present, 31,000 bed volumes. Significant difference in bed volumes to breakthrough was however observed when S2O32- was more than 10 times greater than the ClO4- concentration. The TGAC capacity to remove ClO4- decreased to 17% and 50% when S 2O32- concentration was 250μg/L and 1mg/L respectively. Results to date suggest that S2O3 2- competes with ClO4- for adsorption sites and chlorination can be a useful pre-treatment method for removing this reduced competitive sulfur specie from groundwater before it enters the TGAC column.