The success of a freeze start of a polymer electrolyte fuel cell depends on the state and spatial distribution of the water produced in the porous structures of the membrane electrode assembly (MEA), namely, the gas diffusion and catalyst layers. To improve understanding of performance loss that occurs during freeze starts, the water/ice phase is imaged in the MEA by means of X-ray tomographic microscopy at temperatures between −10 and −20 °C. As the duration of the isothermal freeze starts can be as short as 20 s before a performance drop occurs, the acquisition time for one tomographic scan is reduced to only 4.9 s. Interpretation of the images and the voltage transients show three different mechanisms for the performance drop. The main difference between the three mechanisms is the lack or appearance of supercooled water in the gas diffusion layer. Fast X-ray imaging is well suited to solve this challenge.