The physical immobilisation of a range of crown ethers within hydrogel membranes of a range of water contents has been examined. The efficiency of the immobilisation process increases with increasing size of the crown and decreasing water content (and thus mean pore radius) of the hydrogels. Successful occlusion was achieved with dicyclohexano-18-crown-6 in a poly(2-hydroxyethyl methacrylate) membrane containing 10% ethyleneglycol dimethacrylate crosslinking agent. Of various ions examined with this modified membrane, K+ was subject to the greatest modulation of its transport behaviour; this being characterised by a substantial initial induction period followed by a steady state rate that never attained the value achieved with unmodified membranes of a similar water content. This behaviour is quite novel in comparison with ionophore modulated transport in hydrophobic membranes but bears some resemblance to that of gas permeation in glassy polymers. (C) 2000 Elsevier Science Ltd. All rights reserved.