Potassium persulfate was compared with hydrogen peroxide as oxidant in pressurized hot water. Oxidant tin excess) and organic model pollutants (phenol, 2,3-dichlorophenol, and m-cresol, c = 0.3-0.5 mM each) were introduced to the reaction capillary with and without preheating. Reaction time was 4-57 s, temperature 80-400 degrees C, and pressure 220-310 bar. Without preheating, potassium persulfate was clearly more efficient than hydrogen peroxide in oxidizing the model compounds: for example, with potassium persulfate, removal percentages close to 100 were obtained in 55 s at 110 degrees C, whereas with hydrogen peroxide the required temperature to obtain similar results in any reaction time was nearly 300 degrees C. Total organic carbon was removed more effectively by potassium persulfate under mild conditions. Overall, oxidation efficiencies were decreased when capillaries were preheated, especially with potassium persulfate as oxidant. Corrosion, measured as Ni concentration of the effluent, was more severe with potassium persulfate than hydrogen peroxide.