Scaling up microbial fuel cells (MFCs) will require more compact reactor designs. Spacers can be used to minimize the reactor size without adversely affecting performance. A single 1.5 mm expanded plastic spacer (S1.5) produced a maximum power density (97 +/- 26 mW m(-2)) that was similar to that of an MFC with the cathode exposed directly to air (no spacer). However, a very thin spacer (1.3 mm) reduced power by 33%. Completely covering the air cathode with a solid plate did not eliminate power generation, indicating oxygen leakage into the reactor. The S1.5 spacer slightly increased columbic efficiencies (from 20% to 24%) as a result of reduced oxygen transfer into the system. Based on operating conditions (1000 Omega, CE = 20%), it was estimated that 0.9 L h(-1) of air would be needed for 1 m(2) of cathode area suggesting active air flow may be needed for larger scale MFCs. (C) 2012 Elsevier Ltd. All rights reserved.