A titanium Porous Transport Layer (PTL) is usually used at the anode side of PEM water electrolyzers to ensure both the gas/water transport and the electric charges transfer. In this paper, four different sintered Ti powder PTLs were characterized to determine some properties, such as the pore size distribution, the porosity, and the permeability. Their influence on the electrolysis performance was investigated by using a 30 cm2 segmented cell which allowed measuring the current density distribution, while controlling temperature and pressure conditions. For a better understanding, in-situ techniques such as the Polarization Curves and the Electrochemical Impedance Spectroscopy (EIS) were used. A local characterization of mass transport limitations caused by oxygen saturation was carried out, paying special attention to the pressure influence when using a PTL with very small pores. The results showed that current density heterogeneities can be explained by microstructure changes along the PTL. The optimal geometric characteristics of the PTL depend not only on the operating conditions such as current density, pressure, and temperature but also on the catalyst layer properties. A new model for the constriction resistance between the catalyst layer and the PTL was proposed.