Microaerobic hydrolysis-acidification (MHA)-anoxic-oxic (A/O) processes were developed to treat actual petrochemical wastewater. The results showed that the overall COD removal efficiency was 72-79% at HRT = 20 h, and MHA accounted for 33-42% of COD removal, exhibiting good efficiency of acidogenic fermentation. Ammonium removal was more than 94%. The main pollutants in the influent were identified to be benzene, ketone, alcohols, amine, nitrile and phenols by GC-MS, and the majority of pollutants could be removed by MHA-A/O treatment. Proteobacteria was the most dominant bacteria in the system, accounting for more than 55% of the reads. The predominant genera in MHA, anoxic and oxic reactors were Anaerolineaceae and Sulfuritalea, Lactococcus and Blastocatella, and Saprospiraceae uncultured and Nitrosomonadaceae, respectively. This treatment system exhibited good performance in degrading the complex compounds in the petrochemical wastewater. (C) 2015 Elsevier Ltd. All rights reserved.