Activated carbon (AC) is a high performing and cost effective catalyst for oxygen reduction reactions (ORRs) of air-cathodes in microbial fuel cells (MFCs). Acidic (HNO3) and alkaline (KOH) pretreatments on AC at low temperature (85 degrees C) are conducted to enhance the performance of MFCs. The alkaline pretreatment increased the power density by 16% from 804 +/- 70 to 957 +/- 31 mW m(-2), possibly due to the decrease of ohmic resistance (from 20.58 to 19.20 Omega) and the increase of ORR activities provided by the adsorbed hydroxide ion and extra micropore area/volume after alkaline pretreatment. However, acidic pretreatment decreased the power output to 537 +/- 36 mW m(-2), which can be mainly attributed to the corrosion by adsorbed proton at the interface of AC powder and stainless steel mesh and the decreased pore area. (C) 2013 Elsevier Ltd. All rights reserved.