The presence of water within the polymeric ionic conductor and catalyst layers is indispensable for proton exchange membrane fuel cell (PEMFC) operation. The water transport within the membrane is always a balance between at least two competing mechanisms (water back-diffusion from the cathode to the anode, and electro-osmotic drag from the anode to the cathode). We present an experimental technique for the analysis of water transport within and through the membrane. Mass spectrometry was used to investigate diffusion and electro-osmotic drag in real time (detection rates < 100 ms). We present data on the net water transport resulting from the analysis at room temperature in Nafion115.