Hydroxyl radical (•OH) water demand is a key parameter which impacts the design and operation of UV/H2O2 process for water treatment. Long-term monitoring of the •OH water demand in water sources used for drinking water production indicated significant seasonal variations of this parameter (1.59 × 104 to 4.98 × 104 s-1), which coincided with the occurrence of algal blooming events. Pilot-scale tests at a drinking water treatment plant confirmed that the UV/H2O2 process performance for contaminant removal is predictable when the •OH water demand is accurately determined through a validated experimental method. A predictive tool was developed to identify the optimum operating conditions of the UV system with the UV/H2O2 process and it was used to demonstrate the significant impact of seasonal variations of •OH water demand on the operating costs.