Here we report an experimental in situ analysis of internal gas crossover leakage in single high-temperature polymer electrolyte membrane fuel cells. The study is based on the analysis of the exhausts from the anode and the cathode of the fuel cell during operation. An abnormal crossover rate across the membrane of the investigated fuel cell was detected, indicating the presence of an internal leakage. The internal flux shows linear dependence on the pressure difference between fuel cell compartments, attesting for permeation as the driving force. When the average cathode pressure is higher than the anode pressure, air crossover is measured. Conversely, hydrogen crossover is measured when the anode pressure is higher than the cathode. The effects on fuel cell performance are evaluated under air or hydrogen crossover conditions. Under both conditions, crossover leakage causes a significant increase in hydrogen consumption that reduces fuel recovery from anode exhaust. (C) 2012 Elsevier B.V. All rights reserved.