Second-sphere coordination was investigated spectrophotometrically and electrochemically with various ruthenium-ammine complexes with crown ethers in acetonitrile solution. Spectrophotometric results revealed that the acidity of the ammine ligands of the complex, which depends on the valence of the metal center, predominantly affects the second-sphere coordination of the crown ether to the ruthenium-ammine complexes. This fact explains the difference in the stoichiometry between the adducts of ruthenium(II) and ruthenium(III) complexes with 18-crown-6 ether. It was found that the number of ammine ligands, the pi-electron acceptability of ancillary ligands of the complex, and the flexibility of the crown ether ring affected the change in the redox potential of the complexes caused by second-sphere coordination of crown ethers.