The objective of the laboratory study is to examine the conditions under which transition metal ions (e.g., ferrous ion, Fe 2+) could activate the persulfate anion (S 2O 8 2−) to produce a powerful oxidant known as the sulfate free radical (SO 4 − ) with a standard redox potential of 2.6 V. The SO 4 − is capable of destroying groundwater contaminants in situ such as trichloroethylene (TCE). Experiments using Fe 2+ as an activator under various molar ratios of S 2O 8 2−/Fe 2+/TCE in an aqueous system indicated that partial TCE degradation occurred almost instantaneously and then the reaction stalled. Either destruction of SO 4 − in the presence of excess Fe 2+ or the rapid conversion of all Fe 2+ to Fe 3+ limited the ultimate oxidizing capability of the system. Sequential addition of Fe 2+ in small increments resulted in an increased TCE removal efficiency. Therefore, it appeared that Fe 2+ played an important role in generating SO 4 − . An observation of oxidation–reduction potential (ORP) variations revealed that the addition of sodium thiosulfate (Na 2S 2O 3) to the ferrous ion activated persulfate system could significantly decrease the strong oxidizing conditions. It was hypothesized that the thiosulfate induced reducing conditions might convert Fe 3+ to a lower valence state of Fe 2+, making the Fe 2+ available to activate persulfate decomposition. The sequential addition of thiosulfate (S 2O 3 2−), after the initial stalling of ferrous ion activated persulfate oxidation of TCE, resulted in an improvement in TCE removal. The ferrous ion activated persulfate–thiosulfate redox couple resulted in fairly complete TCE degradation in aqueous systems in a short time frame. In soil slurry systems, TCE degradation was slower in comparison to aqueous systems.