A new class of crown ethers containing heterocyclic units has been designed to develop synthetic NH4+-selective ionophores whose resulting potentiometric properties in solvent polymeric membranes are superior or at least comparable to those of nonactin. It was found that the derivatives of thiazole containing dibenzo-18-crown-6 (1; TDB18C6) incorporated in PVC-based membranes provide enhanced NH4+ selectivities over alkali metal cations, especially over Na+, compared to those of the nonactin-doped membranes: for example, the selectivity coefficients, log K(POT)NH4+J (J = Li+, Na+, K+), for the 2-nitrophenyloctyl ether (NPOE) plasticized PVC membranes doped with the hexyl chain-substituted TDB18C6 (2) were -4.9, -3.9, and -1.3, while those for the same type of membranes with nonactin -4.4, -3.0, and -1.0, respectively. Unlike other synthetic NH4+-selective neutral carriers reported to date, TDB18C6-type compounds result in potentiometric performance highly comparable to that of the nonactin-based ones except their slightly higher detection limits (approximately 3 x 10(-6) vs 7 x 10(-7)). The complex formation between TDB18C6 and NH4+ was identified from 1H NMR spectra. The 1:1 complex formation constant of TDB18C6 with NH4+ in solvent polymeric membranes estimated using the method suggested by Bakker et al. and the NH4+ selectivity coefficient over K+ (K(POT)NH4+,K+) are approximately 200 times smaller than that of nonactin (1.8 x 10(7) vs 3.6 x 10(9)).