The oxidation of aqueous monochlorobenzene (MCB) solutions using thermally-activated persulfate has been investigated. The influence of reaction temperature on the kinetics of MCB oxidation was examined, and the Arrenhius Equation rate constants at 20°C, 30°C, 40°C, 50°C, and 60°C for MCB oxidation performance were calculated as 0, 0.001, 0.002, 0.015, 0.057 min−1, which indicates that elevated temperature accelerated the rate. The most efficient molar ratio of persulfate/MCB for MCB oxidation was determined to be 200 to 1 and an increase in the rate constants suggests that the oxidation process proceeded more rapidly with increasing persulfate/MCB molar ratios. In addition, the reactivity of persulfate in contaminated water is partly influenced by the presence of background ions such as Cl−, HCO3−, SO42−, and NO3−. Importantly, a scavenging effect in rate constant was observed for both Cl− and CO32− but not for other ions. The effective thermally activated persulfate oxidation of MCB in groundwater from a real contaminated site was achieved using both elevated reaction temperature and increased persulfate/MCB molar ratio.