Comparison of advanced oxidation processes (AOPs) can be difficult due to physical and chemical differences in the fundamental processes used to produce (OH)-O-. radicals. This study compares the ability of several AOPs, including ozone, ozone+H2O2, low pressure UV (LP)+H2O2, and medium pressure UV (MP)+H2O2 in terms of energy required to produce (OH)-O-. radicals. Bench scale (OH)-O-. radical formation data was generated for each AOP using para-chlorobenzoic acid (pCBA) as an (OH)-O-. radical probe compound in three waters, Lake Greifensee water, Lake Zurich water, and a simulated groundwater. Ozone-based AOPs were found to be more energy efficient than the UV/H2O2 process at all H2O2 levels, and the addition of H2O2 in equimolar concentration resulted in 35% greater energy consumption over the ozone only process. Interestingly, the relatively high UV/AOP operational costs were due almost exclusively to the cost of hydrogen peroxide while the UV portion of the UV/AOP process typically accounted for less than 10 percent of the UV/AOP cost and was always less than the ozone energy cost. As the (OH)-O-. radical exposure increased, the energy gap between UV/H2O2 AOP and ozone processes decreased, becoming negligible in some water quality scenarios. (c) 2006 Elsevier Ltd. All rights reserved.