4-Chloro-3-methylphenol (c = 2.0 mM), representing a model pollutant, was oxidized in pressurized hot water and in supercritical water in a continuous flow system. Potassium persulfate was used as oxidant in concentrations of 8.0 and 40.0 mM. Contact times (reaction times) were 3-59 s, temperatures 110-390 degrees C, and pressures 235-310 bar. A wide temperature range was tested to determine the range over which potassium persulfate can be used effectively. Good oxidation efficiencies for 4-chloro-3-methylphenol were obtained at both oxidant concentrations and with short contact times at temperatures clearly underthe critical temperature of water; total organic carbon content of the effluent was low under optimized conditions. Corrosion, measured as nickel and chromium concentrations of the effluent, was more severe at oxidant concentration of 40.0 mM. Sulfate was present in the effluent in high concentrations. Sulfate is the limiting factor in the use of potassium persulfate in wastewater treatment and requires further water treatment.