This paper reports the first theoretical modeling study of air-breathing microfluidic fuel cells (MFCs). The model is based on a semi-empirical Graetz-Damkohler (Gz-Da) analysis. The theoretical formulas derived clearly demonstrate the effects of the MFC design and operational parameters on the electrochemical activities of MFCs in quantitative detail. The modeling analysis shows that the electrode kinetics have significant effects on the trade-off relationship between the fuel utilization and current density. Moreover, the analysis reveals that the co-laminar flow of MFCs limits the fuel utilization considerably. (C) 2012 Elsevier B.V. All rights reserved.