The effect of dissolved redox mediators on denitrification by Paracoccus versutus sp. GW1 were studied with alpha-AQS, AQS, 1,5-AQDS, AQDS and 2,7-AQDS. AQDS presented the best catalytic effect which increased denitrification rate by 1.5-fold with 0.17 mmol L-1. Denitrification rate was found to be pseudo-zero order with 0-0.32 mmol L-1 AQDS (k = 19.874C(AQDS) + 10.301 (R-2 = 0.9984)). The accelerating mechanism for respiratory chain was also explored by using metabolic inhibitors. Addition of AQDS eased the inhibitions of rotenone, dicumarol, sodium azide on respiratory chain, and enhanced the nitrite accumulation during bio-denitrification process. AQDS exhibited slight catalytic effect on nitrite denitrification. These results allowed us to deduce the accelerating site of AQDS on the denitrification respiratory chain. Increased nitrate denitrification by AQDS might mainly attribute to the accelerated electrons transfer from NADH to nitrate reductase via complex I and ubiquinone/ubiqunol. This study provided further understanding of the mechanism of redox mediators on denitrification. (C) 2013 Elsevier Ltd. All rights reserved.