Polyethyleneterephthalate track-etched membranes with a pore diameter of 650 nm were functionalized via surface-initiated atom transfer radical polymerization with grafted poly(2-hydroxyethylmethacrylate). Grafted chain length and density were varied. Superparamagnetic nanoparticles (Fe3O4; core diameter 15 nm) were selectively covalently coupled to the end groups of the grafted chains. The membranes were characterized by grafting degree, X-ray photoelectron spectroscopy, electron microscopy, zeta potential and pore size in dry state via gas flow/pore dewetting permporometry. The results confirmed that all functionalization steps were well controlled. Water permeability measurements allowed estimation of the hydrodynamic pore diameter of the membranes, and, hence, the hydrodynamic polymer layer thickness on the pore walls. The water permeability of the nanoparticle hybrid membranes was then measured in a static or an alternating external magnetic field. Significant and reversible decreases of permeability were observed, with the largest effects for membranes with high polymer grafting density and long polymer chains (hydrodynamic layer thickness up to 100 nm). The maximum change in effective pore diameter was only 6%. However, the estimated change of swollen polymer layer thickness (originally between 60 and 100 nm) was up to 13 nm. The functionality of the membranes can be tuned by variations of straightforward parameters such as pore size or grafted chain lengths. The study is also relevant as a model system for altering the effective thickness of grafted polymer layers on a surface by an external magnetic field for other applications, for instance in microfluidic systems. (C) 2012 Elsevier B.V. All rights reserved.