New 25,27-dialkoxycalix-arenecrown-5 conformers 8, 10, and 11 have been synthesized and studied. The compounds 8a and 8b, fixed in 1,3-alternate structure, have been obtained in 57 and 40% yield, respectively, by reaction of the corresponding 25,27-dialkoxycalixarenes 7a-b with tetraethylene glycol di-p-toluenesulfonate in the presence of Cs2CO3. The cone 10a and 10b and the partial cone 11 conformers were obtained by selective demethylation of the 25,27-dimethoxycalixarenecrown-5 (6a) and subsequent dialkylation with NaH/DMF and KOtBu/THF, respectively. In the solid state (X-ray), compound 6a adopts a flattened cone conformation, which is also found to be most abundant in CD3CN and CD3OD solution. Upon complexation with potassium picrate compound 6a was converted quantitatively into the 1,3-alternate conformation. All new ligands synthesized were used in the extraction of alkali metal cations from H2O into CHCl3, and as active components in supported liquid membranes and in chemically modified field effect transistors. Results were compared to those obtained with with the natural antibiotic valinomycin 1. All ligands showed high selectivity for potassium. Ligand 8a, fixed in the 1,3-alternate conformation, is more selective than valinomycin and shows the highest K+/Na+ selectivity known so far.