The 133Cs NMR method was used to study the complexation of a series of crown ethers with caesium cations in NN-dimethylformamide (DM F). A competitive' 13Cs NMR technique was also used to probe the complexation of rubidium cations with the same crown ethers. The ligand 18-crown-6 forms the most stable complexes with caesium and rubidium cations. The effect of the addition of the substituent group(s) on the crown ring and the effect of the replacement of the oxygen donor atoms with nitrogen and sulfur atoms in 18-crown-6 were studied. The results show that the complex formation constants with Cs + cation increase through the series: dithia-18-crown-6 ⪡ diaza-18-crown-6 < dibenzo-18-crown-6 < dicyclohexano-l8-crown-6 < 18-crown-6. The trend with rubidium metal ion is: diaza-l8-crown-6 < dicyclohexano-l8-crown-6 < dibenzo-l8-crown-6 < 18-crown-6. Among the ligands studied, only 18-crown-6 and dicyclohexano-24-crown-8 form both 1 : 1 and 2 : 1 (ligand : cation) complexes with the caesium cation in DMF.