A microporous PTFE/carbon sublayer between the carbon paper and the catalyst layer played a crucial role in PEM fuel cells. Such a layer reduced the difference among different carbon paper types made by different manufacturers or by the same manufacturer at different times. When such a sublayer was present, the carbon paper seemed to function merely as a support that provided the mechanical strength for the sublayer. A sublayer was extremely useful if the carbon paper was prone to flooding. Sublayers of different thickness containing 24, 35 and 45% PTFE were tested. A four-cell stack could operate stably at a current density of 145 mA/cm(2) using unhumidified air and hydrogen when sublayered carbon paper was employed.