The effects of Na+, K+ and Li+ cations on the fluorescence spectra of benzocrown-5, benzocrown-6 and dibenzocrown-6 were investigated in acetonitrile. The alkali cation role observed was usually the complexation-enhanced quenching fluorescence effect (CEQF) in acetonitrile due to the increased fluorescence quenching rate of the complexed fluoroionophore. The association constants for 1:1 stoichiometry In K-a have been obtained using the relationship 1/K-a[L(O)] = (1 - P)(2)/P. It was shown that the preferential interaction rule of compatibility of cationic radii and macrocyclic ring size is in excellent agreement with the association constants obtained by fluorescence spectroscopy. The order of In K-a found for benzocrown-5 complexation was Li+ > Na+ > K+ and K+ > Na+ > Li+ for benzocrown-6 in acetonitrile.