BACKGROUND: To investigate the effects of pH on performance and anodic microbes of MFCs (microbial fuel cells), double-chamber MFCs were fed-batch operated at four different values of initial pH(4, 5, 6, and 7) and the changes in anodic microbes (species and appearance) were studied. RESULTS: Lower voltage outputs (232-284 mV vs. 311-339 mV) and power generation (95-116 mW m(-2) vs. 182-237 mW m(-2)) with faster COD removal were obtained under acidic pH conditions. Simplicispira, Variovorax, Comamonas, and Acinetobacter were the major communities under acidic conditions, while Chlorobi, Aquaspirillum, and Sphingomonas were in the majority under neutral conditions. Anodic biofilms cracked and reduced at pH <= 5. MFCs operated at pH 4 failed to recover optimal electricity generation when pH was readjusted to 7. There were significant correlations between the time-course pH changes (anodic and cathodic) and voltage outputs of the MFC under neutral conditions. CONCLUSIONS: Injured anodic microbes and biofilms may be the reason for decreased MFC performance under acidic conditions. pH <= 4 may cause long-term, even irreversible reduction to MFC performances. (C) 2011 Society of Chemical Industry