This study investigated the effect of manganese [Mn(IV)] amendment on the anaerobic biodegradation of four mixed PAHs, namely fluorene (FI), phenanthrene (Phe), fluoranthene (Flua) and pyrene (Pyr) under low-oxygen condition, with and without the inoculation of enriched PAH-degrading bacterial consortia, in mangrove sediment slurries. The results revealed that the addition of Mn(IV) significantly inhibited PAH biodegradation, the rate of which was about 31-70% lower than the one of the groups without Mn(IV) addition. The amendment of Mn( IV) also showed adverse effect on the population size of enriched PAH-degrading bacteria and bacterial activity. The analysis results on the concentrations of Mn(II) and Mn(IV) indicated that Mn(IV) was converted to Mn(II) fast, the latter was the predominate manganese form in the mangrove sediment slurries through the whole experimental period. The Mn(II) toxicity to microorganisms was considered the main reason for inhibition of the PAH-biodegradation. On the other hand, the inoculation of the enriched PAH-degrading consortia significantly enhanced the biodegradation rates of all four PAHs, and the biodegradation rates of 3-rings (El, Phe) and 4-rings (Flua, Pyr) PAHs were enhanced by 14-15% and 21-34%, respectively. (C) 2011 Elsevier By. All rights reserved.