A technique, called differential resistance analysis, was developed to determine the bulk conductivity of protonic ceramic electrolyte in hydrogen/air fuel cells under load. For these experiments, identical specimens of the protonic ceramic BaCe0.9Y0.1O3-alpha (BCY10) were prepared with thicknesses of 240, 560, 790, and 1150 mum with thin film platinum electrodes. The current-voltage (I-V) characteristic curves for each specimen were obtained between 600 and 800 degreesC, and the slope of each I-V curve was determined in the ohmic region between 10 and 20 mA, giving the total effective area specific resistance (ASR cell) of the cell under load as a function of temperature. The bulk electrolyte resistivity was found by taking the difference in resistance of two cells divided by the difference in electrolyte thickness. The bulk conductivity of the electrolyte measured at 1000 K by this technique was 5 mS/cm or less, depending on the overall electrolyte thickness, much lower than the values obtained by impedance spectroscopy. Also, the activation energy for bulk conduction was found to be higher than expected for pure protonic transport. This paper attempts to correlate the two measurement techniques and explain the apparent discrepancies. (C) 2003 Elsevier B.V. All rights reserved.