Electrolysis with polymer electrolyte membranes (PEMs) plays an increasingly important role in the development of inconsistent renewable energy technologies and seasonal storage. An effect that reduces the efficiency of PEM electrolysis is the mass transport limitation (MTL), which occurs at higher current densities and leads to a sudden increase in cell potentials. The oxygen generated on the anode side prevents the water from being supplied to the catalyst. Neutron-based imaging (neutron visualization techniques) make it possible to visualize mass transfer processes in the porous transport layer (PTL). When PTL materials are varied and operating modes used, it is possible to investigate the critical point at which the MTL is generated. This paper presents the results of neutron radiography measurements. In the course of the measurements, we have observed PEM electrolysis cells in operation, using different materials as anode PTL. The PTLs are metal plates made of sintered titanium particles, as well as titanium fiber. During the measurements, it was possible to visualize the water-gas distribution in the cell during operation in order to understand the mechanisms of mass transport.