PEROXONE is an advanced oxidation process for water treatment which is generated by combining ozone and hydrogen peroxide. In this study, surface water supplies were seeded with viableGiardia muriscysts and disinfected by ozone and PEROXONE in the pretreatment columns of a 22.7 L/min pilot plant. Inactivation was examined in two source waters under conditions of varying applied ozone doses (0.5–4.0 mg/L), with and without the addition of hydrogen peroxide; at increasing contact times (3, 6, 9, and 12 min); and with and without added high turbidity (10 and 50 NTU). Approximately 108viableG. muriscysts were injected into the ozone contactor-column influent. The cysts were collected at the ozone contactor-column effluent and concentrated, and viability was then determined usingin vitroexcystation.