In this paper, the quenching of hydrogen peroxide by catalase, sodium hypochlorite, sodium thiosulfate and sodium sulfite, prior to UFC testing, was investigated. Sodium hypochlorite, sodium thiosulfate and sodium sulfite were found to be unsuitable for quenching H2O2 residuals because the procedures are time-consuming and complicated in that they require potentially multiple measurements of the peroxide and chlorine residuals. In contrast, quenching of peroxide with catalase is a simple procedure. Catalase doses of less than 0.2 mg/L were found to have no impact on DBP (TTHM, HAA and aldehyde) formation in the UFC test, and the time that was needed to quench 100 mg/L peroxide (room temperature, pH 8.3) was less than 10 min. In addition, peroxide was found to react with DPD reagents that are used to measure chlorine residuals, a phenomenon that may lead to falsely high chlorine residuals in the UFC test.