The Electra-Fenton (EF) process has shown great potential in refractory wastewater treatment. However, its efficiency is highly pH-dependent and the high reagent costs for pH adjustment greatly limit its large-scale application. This study used a bench-scale EF reactor to study pH variation during the treatment of an acrylic fiber wastewater. The appearance of pH inflection points (pH IPs), i.e., sudden changes in pH, under different H2O2 dosages, current densities, and ratios of [Fe-Theo(2+)][H2O2] (R-Fe center dot H2O2), were investigated. The pH IPs appeared on the time scale of 5-7.5 min, and were favored at decreased H2O2 dosages (2.5-1.0 mM), increased current densities (3-4.5 mA/cm(2)), and elevated R-Fe center dot H2O2 (0.1:1-0.2:1). The appearance of pH IPs was indicative of the exhaustion of H2O2 and a significant decrease of COD degradation, as expressed by the pseudo-first order kinetic constant (k) of COD degradation. Besides this, the substitution of Fe(OH)(3) by Fe(OH)(2-) and Fe(OH)(2)(-) occurred after pH IF appearance, and the dominant mechanism changed from electro-chemical oxidation (EO) to electro-coagulation (EC) accordingly. In the presence of sufficient H2O2, the formation of Fe(OH)(3) barely affected pH variation. After H2O2 exhaustion, i.e., the appearance of pH IPs, dissolved O-2 rapidly oxidized Fe2+ to Fe3+, the hydrolysis of which contributed to immediate pH increase thereafter. The optimization of the EF process may be achieved by adjusting H2O2 doses and the control of pH IP appearance. (C) 2015 Elsevier B.V. All rights reserved.