In this paper, the humidification issues of proton exchange membrane (PEM) fuel cells were experimentally analyzed using three fuel cell systems (FCSs) based on stacks of different sizes (2.4, 6.2 and 14 kW). Both internal and external humidification strategies were considered. External humidification was performed on the air stream using the following techniques: air saturation at different temperatures (bubbler), water injection into the cathode manifold and heat and mass exchange by selective polymeric membranes. The internal humidification analysis focused on the self-humidification approach. The effect of humidification strategies on membrane hydration was evaluated by analyzing the stack performance and its power loss rate. The external humidification strategy was effective at most operative conditions, but it exhibited limitations at typical conditions which favored membrane dry-out (i.e., low load and high stack temperature). At a high load and temperature, the external humidification was effective when the saturation temperature of the inlet air stream was maintained at values close to the stack temperature (temperature difference < 5 K). The self-humidification technique was shown to be the most practical choice for application in hybrid fuel cell vehicles, though it requires accurate control of the stack temperature profile in the range of 303-328 K for normalized powers (P/P-max.) between 20 and 90%. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.