This study investigated the effect of pH on the formation of disinfection by-products (DBPs) during preoxidation with ferrate(VI) Fe(VI)O-4(2-) and subsequent chlorination of either a source water from a treatment plant, or a representative solution of natural organic matter from the Suwannee River NOM (SRNOM). The studied DBPs include trihalomethanes (THMs), chloral hydrate (CH), haloacetonitriles (HANs), and trichloronitromethane (TCNM). The results show that increasing pH from 5.0 to 9.0 increased THMs formation during chlorination of source water and Fe(VI) pre-oxidation generally decreased the concentration of THMs, except pH 9.0. The formation of CH and TCNM were not greatly influenced by pH in chlorination. But Fe(VI) pretreatment before chlorination enhanced CH and TCNM formation in acidic pH while lessened their formation in alkaline pH. HANs were detected in acidic pH whereas almost no formation of HANs was observed in basic pH. Similar experiments were conducted with SRNOM water to assess the variation of DBPs at different pHs with/without Fe(VI) pre-oxidation. Reactivity, products of fractions, and moieties of organic matter involved in Fe(VI) per-oxidation and subsequent chlorination reasonably explained the trends of the formation of DBPs under acidic to basic conditions. (C) 2015 Elsevier B.V. All rights reserved.