Water uptake and electro-osmosis are investigated to improve the understanding and aid the modeling of water transport in proton-exchange membrane fuel cells (PEMFCs) below 0 degrees C. Measurements of water sorption isotherms show a significant reduction in the water capacity of polymer electrolytes below 0 degrees C. This reduced water content is attributed to the lower vapor pressure of ice compared to supercooled liquid water. At -25 degrees C, 1100 equivalent weight Nafion in equilibrium with vapor over ice has 8 moles of water per sulfonic acid group. Measurements of the electro-osmotic drag coefficient for Nation and both random and multiblock copolymer sulfonated poly(arylene ether sulfone) (BPSH) chemistries are reported for vapor equilibrated samples below 0 degrees C. The electro-osmotic drag coefficient of BPSH chemistries is found to be similar to 0.4. and that of Nation is similar to 1. No significant temperature effect on the drag coefficient is found. The implication of an elcctro-osmotic drag coefficient less than unity is discussed in terms of proton conduction mechanisms. Simulations of the ohmically limited current below 0 degrees C show that a reduced water uptake below 0 degrees C results in a significant decrease in PEMFC performance. (C) 2008 The Electrochemical Society.