In this study, the local behavior of an electrochemical hydrogen compressor (EHC) was investigated. A local dehydration of the polymer electrolyte membrane (PEM) was experimentally observed, due to the unbalanced contribution of the electro-osmosis flow and the back diffusion of water across the membrane. Such operating heterogeneities can significantly affect the overall efficiency of an EHC. A pseudo 2D model was developed along with experimental studies in order to estimate the physical parameters enhancing the overall efficiency of the system. 1. Introduction Electrochemical hydrogen compressor (EHC) has proven to be a valid solution to compress hydrogen. Even though pressures up to 1000 bar can be reached using an EHC [1], very high pressure gradients can cause issues related to sealing and the diffusion of dissolved hydrogen molecules across the membrane from the high pressure cathode to the low pressure anode, decreasing the overall efficiency. Rohland et al. [2] showed that hydrogen permeation across the membrane is a function of both the pressure gradient and the temperature. In particular, they showed that the higher the EHC temperature, the higher the hydrogen permeation. Grigoriev et al. [3] showed that it is possible to compress hydrogen from atmospheric pressure to 48 bar in a single step with an energy consumption of 0.3 kWh/Nm 3 and an efficiency around 50%. This value is in average higher than those obtained with mechanical compressors [4]. Water management is an important issue in an EHC. As in proton electrolyte membrane fuel cells (PEMFCs), the polymer electrolyte membrane (PEM) has to be hydrated in order to enhance its proton conductivity. Nevertheless, contrarily to PEMFCs, water is not a reaction product in an EHC, thus it needs to be fed along with hydrogen in order to preserve the optimal hydration degree of the membrane. Onda et al. [5] found that both the hydrogen concentration in the gas distribution channels and the current density distribution decrease along the channel direction during operation. This behavior was found to be directly related to the water transport across the membrane. Heterogeneities in the distribution of the electric resistances are the main consequence of the unstable water flow across the membrane: specifically, the local dehydration of the membrane can lead to an increase of the electric resistance of the system, which in turn makes the current density decreasing. In this study, the local behavior of an EHC is investigated and the effect of the humidity of feed gas, temperature and membrane thickness was evaluated and discussed. A pseudo 2D model taking into account the overall mass and energy balance occurring in the EHC, as well as the heterogeneities introduced above, was developed along with the experimental investigation.