Proper gas and water management are essential to achieving and maintaining high power output in a PEM fuel cell stack. Previous experimentation with a small oxygen fuel cell stack has demonstrated that the use of an active gas management (AGM) system to control individual exhausts improved and sustained high power with oxygen operation. The active gas management system has now been employed in small and large air cathode stacks of five and six cells. The use of the AGM system increased the small five-cell stack's power output from 38.4 to 50.4 W and increased the large six-cell stack's power output from 260 to 350 W. Both large and small stack's demonstrated a 30% power increase after accounting for parasitic losses. (C) 2004 Elsevier B.V. All rights reserved.