Pilot-plant studies were conducted in two source waters to determine the effects of predisinfection with ozone alone and with a combination of hydrogen peroxide and ozone (PEROXONE) on the production of assimilable organic carbon (AOC) compounds. Colorado River water (CRW) and State project water (SPW) from Northern California were treated with ozone alone at applied dosages ranging from 1 to 4 mg/L and with PEROXONE at hydrogen peroxide/ozone (H2O2/O3) ratios of 0.05, 0.10, 0.20, and 0.30. Ozonation of CRW with applied dosages of 1.0,2.0, and 4.0 mg/L increased AOC concentrations from 70μg C/L to 275, 350, and 224 μg C/L, respectively. Ozonation of SPW with an applied dosage of 4.0 mg/L elevated AOC concentrations from 70 to 522μg C/L.