Abstract: As a measure of water integrity in water treatment and distribution systems, stabilized hydrogen peroxide is used as the secondary disinfectant. The concentration of hydrogen peroxide is monitored throughout the system and, if needed, additional hydrogen peroxide is injected into the system to maintain the level at established standard. An apparatus and method for measuring hydrogen peroxide concentration in water to an accuracy of 0.1 mg/L within water treatment and distribution systems comprises a colorimetric assay method to determine hydrogen peroxide concentration. The assay is monitored spectophotometrically at a desired wavelength. Each sample is corrected relative to a control sample and hydrogen peroxide concentration determined with respect to a standard curve.