The synthesis of new fluorescent indicators for Na + , Benz-15-5 or Diaz-15-5, and for K + , Benz-18-6 or Diaz-18-6, respectively, is reported. They consist of a benzocrown or a diazacrown ether linked to a pyrazoline fluorophore substituted with carboxylate groups to ensure water solubility. They were obtained via reaction of an α,β-unsaturated ketone with an aryl hydrazine. The fluorescence intensity of Benz-15-5 is nearly insensitive to Na + . Benz-18-6 displays a four-fold selectivity for K + over Na + , with a dissociation constant of 3.6 mM for the K + :indicator complex. Diaz-15-5 and Diaz-18-6 have only a weak affinity for monovalent cations, so that they only can be used for the determination of high Na + and K + concentrations.