Fuel cells are gaining increasing attention as portable power sources due to their inherent efficiency advantages. Many aspects of electrode and catalyst behavior within an operating fuel cell, however, are still not well-understood. The major divide between catalyst-based rotating disk electrode experiments and full-cell experiments can lead to disappointing results when a promising catalyst is tested in a fuel cell. Here, using a flowing electrolyte-based microfluidic H-2/O-2 fuel cell with a reference electrode, we demonstrate the ability to analyze single-electrode performance in-situ using a novel analysis protocol that provides kinetic parameters R-ohmic and eta(kinetic) to quantify individual electrode data. Using this protocol, we determine the mass transport and ohmic effects on these kinetic parameters and correlate them with actual fuel cell performance. We also compare the performance of identical electrodes in alkaline and acidic media using both our analytical method and electrochemical impedance. The quantitative parameters are show to predict power density within 5% for measured data and were then used to predict performance for a newly assembled fuel cell, which was accurate within 10% of actual power density inside the measured range. In summary, the analytical method reported here can improve the understanding of in-situ electrode behavior. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.117206jes] All rights reserved.