This study describes the determination and comparison of some cations such as Na+, K+, Ba2+ and Cu2+ in aqueous media by using crown ether dyes immobilized onto thin (0.04 mm) sulfonated kraton polymeric membranes. The studies have been carried out in aqueous solutions first, and in dye-immobilized membrane systems later. The flow-cell system has been found to be more practical in application and had lower response times (30-60 s) as well as detection limits (2.24 x 10(-10)-1.19 x 10(-8) mol l(-1)). The optimum pH range for the formation of ion-dye complexes have been found to be 5 and 9 for the ions above. Since the complexation reactions were reversible it was possible to regenerate the membranes by using HCl solutions and reuse them many times. The same membrane showed high reproducibility and low relative standard deviation. In addition, the interferences of the studied cations to each other and of other various cations during the determinations have also been investigated. (C) 1998 Elsevier Science S.A. All rights reserved.