The Na+ association constants, K-11 and beta(12), of 1/1 and 1/2 stoichiometry of macrocycles of (1,2)dibenzocrown-5, (1,3)dibenzo crown-6, (1,4)dibenzocrown-7, (2,3)dibenzocrown-7, (2,4)dibenzo  -crown-8 and dicyclohexano crown-8 were determined in THF/water, (50/50), mixtures. We used a Methrom Na+ ISE for the potentiometric measurements and applied the 1/(beta(nm)[A(0)(+)])(n+m-1) = (1 - nP(c)) (1 - mP(c)) / P-c equation where the P-c = P/[1 + (m - 1) P] and P is the mole fraction of the complexed cation, [A(n)(+)L(m)] for association constants with linear best fit. Results were in well agreement with the macrocyclic size and cation radii relationships. Measurements showed the role of aromatic groups of macrocyclics in THF/water (50/50) which allowed the macrocyclic molecules more selectively bind Na+ compared to the associations those obtained in water. (C) 2004 Elsevier B.V. All rights reserved.