Stacking of microbial fuel cells (MFC) by connecting multiple small-sized units in a series is used for generating higher power from the MFCs. However, voltage reversal is a critical problem in a serially connected MFC unit. The voltage reversal often occurs when substrate concentration is relatively low in the anodic compartment. Two rectangular individual cells were stacked together in series: MFC1 was fed with 1 g glucose L(-1) throughout the experiment while MFC2 was fed with various concentrations of glucose (0.1, 0.2, 0.3, 0.5 and 0.8 g L(-1)). Voltage reversal occurred when the stack configuration was performed using (1 + 0.1) g glucose L(-1). The stacked configurations with (1 + 0.2, 1 + 0.3, 1 + 0.5 and 1 + 0.8) g glucose L(-1) were operated successfully without the voltage reversal. The maximum powers of 1.88, 2.04, 3.6, 2.5 and 2.18 mW were obtained with the stacked configurations of (1 + 0.2), (1 + 0.3), (1 + 0.5), (1 + 0.8) and (1 + 1) g glucose L(-1), respectively. Except in the stacked configuration with (1 + 0.1) g glucose L(-1), the stacked voltages obtained were similar.