Cation exchange membranes made of blends of sulphonated polyetherether ketone (S-PEEK) and poly(ether sulphone) (PES) were thoroughly characterized with respect to their concentration polarization properties. Current¿voltage curves and chronopotentiometry reveal some extent of membrane heterogeneity. Membranes cast on a glass plate and dried in air are characterized and the current¿voltage curves are determined for each of the two membrane sides (glass side contact and air side contact). Detailed analysis of the plateau length at the limiting current density reveals differences as the orientation of the membrane towards the feed is changed. The plateau length of the air side of the membrane always shows larger values compared to the glass side. Moreover, we discovered that the plateau length of the glass side remains constant whereas the air side value increases over a period more than 500 h approaching a quasi-equilibrium value, asymptotically. These data are the first ones suggesting an influence of orientation on the concentration polarization behavior as well as relaxation phenomena occurring in cation exchange membranes. The paper discusses strategies to gain new fundamental insight in the relationship between membrane morphology and transport properties by for instance microstructuring the surface.