Crossflow filtration of anionic and cationic hydrophobically modified water-soluble polymers was studied using polyether sulphone membranes with varying pore sizes. New data on the permeate flux rates and rejections of these polymer solutions are presented. It was found that the pseudo-gel polarisation concentration increases with increasing polymer solubility. The steady state permeate flux decays linearly with log (MWCO). A minimum molecular weight cut off (MMWCO) can be identified by extrapolating the steady state permeate flux to zero. The MMWCO is well below the molecular weight cut off (MWCO) based on the random coil size of the macromolecule. It is shown that the MMWCO should be based on the size of the small axis of the rod-like extended molecule, which can orientate in the flow field during passage through the pores. This model also explains why the polymer rejection increases with increasing permeate flux.