Air-cathode, microbial fuel cells (MFC) with different anode surface areas were evaluated for simultaneous decolorization of Congo Red and bioelectricity production. Doubling the anode area from 18 to 36 cm(2) increased net power by 150 % (0.16-0.4 mW), normalized power (per anode surface area) by 22 % (88-107 mW m(-2)) and Congo Red decolorization by 163 % (1.6-4.2 mg l(-1) h(-1)). Quadrupling the original anode area induced an additional 5 % increase (up to 4.2 mW) in net power and 174 % increase (up to 11.5 mg l(-1) h(-1)) in Congo Red decolorization; however, normalized power decreased by 85 % (down to 58 mW m(-2)). Increased bacterial attachment could account for both the enhanced power and Congo Red decolorization in larger anode MFCs. The limited effect on power output likely arises from cathode limitation or inefficient utilization of anodes.