Polystyrene networks with elastic chains of known length and containing pendant poly(ethylene oxide) chains selectively attached to the crosslinks were obtained by anionic block-copolymerization. Such networks can exhibit a residual swelling-degree in water that allows easy access to the network-pores. One network with known characteristics was used as a column filling in S.E.C. with water as eluent. Even under high driving pressures, the mechanical strength of this material appears to be satisfactory. In the present example, the domain of selectivity extends from approx. 100–5000.