Activated peroxygens are frequently used as active agents in in-situ chemical oxidation (ISCO) contaminated site remediation applications, and fast and simple quantitative analysis of these species on site is necessary. In this work, the use of a spectrophotometric method based on classic iodometric titration is studied for quantitative analysis of S2O82− and H2O2. Instead of a back-titration step, the absorbance of the yellow iodide colour was measured at 352 nm in the presence of a bicarbonate buffer. A linear calibration curve was obtained from 0 to 0.1 mM for both S2O82− and H2O2. By dilution, the method can be used for all concentrations typically applied in the field. Concerning pH dependence, neutral pH levels caused no significant error whereas pH levels above 8 caused a 9% and 6% deviation from the theoretical peroxygen concentrations. Furthermore, the method showed little dependence on other matrix components, and absorbance was stable (<2% change) for more than a week. Overall, the method proved to be fast and simple, which are important features for a field method.