The environmental occurrence of perchlorate (ClO4) can be related to either natural or anthropogenic sources. Recent studies highlighted the ubiquitous occurrence of natural ClO4 in the environment including wet deposition in the United States. Limited studies have investigated potential mechanisms responsible for natural ClO4 production in the environment. These studies have neither addressed the influence of relevant reaction conditions nor have they evaluated the rates of ClO4 production. The purpose of this study was to determine the comparative yields and rates of ClO4 production from O3 mediated oxidation of Cl, OCl, ClO2, ClO3, and ClO2. The influence of reactant (O3 and ClOx) concentration and pH were evaluated. The comparative rate and efficiency of ClO4 production is generally greater for higher oxidation states of Cl (2.7 to 0.5% for ClO 2/ClO2 and 0.02 to 0.005% for OCl/HOCl oxidation) with the notable exception of ClO3 which does not react with O3. The very slow rate of ClO4 production from Cl (?20 - 109 mM min1) even at elevated O3 and Cl concentrations implies negligible potential for anthropogenic ClO4 formation in process units of water/wastewater systems that use O3 for treatment. Based on results of ClO4 formation from tested Cl species and available literature, we propose a potential formation pathway for ClO4 from Cl with emphasis on the role of ClO2 and higher oxy-chlorine radicals/intermediates (e.g., Cl2O6) in its formation.