The catalytic ability of crown ethers and their complexes with transition metal cations in the desulfurization reaction of mixtures that simulate diesel fuel has been investigated. It has been found that the use of monoaza-15-crown-5-ethers with an admixture of NbCl(5) resulted in a fourfold decrease of the total sulfur content, thereby indicating partial oxidation of benzothiophenes and dibenzothiophenes. The complexation of azacrown ethers with NbCl(5) has been studied by (1)H NMR spectroscopy. A moderate efficiency of adsorption purification procedure using silica and alumina for both initial and oxidized model mixtures has been revealed.