The ring expansion of an azirine has been effected, proceeding through the azirinium salt, generated in situ, as a probable intermediate. The representative azirine, 3,3-dimethyl-2-phenyl-1-azirine, on treatment with acetone and perchloric acid (or fluoroboric acid), formed an oxazolinium salt, 4-phenyl-2,2,5,5-tetramethyl-3-oxazolinium perchlorate (or fluoroborate), identified by infrared, ultraviolet, and nmr spectra, and by hydrolysis to ammonium perchlorate and benzoyl dimethyl carbinol. Using deuterium-labeled acetone, it was possible to determine how this moiety took its place in the five-membered ring product: by a mechanistic pathway following 1.3-bond cleavage in the original azirine. The corresponding base, 4-phenyl-2,2,5,5-tetramethyl-3-oxazoline, was obtained from the perchlorate by treatment with alkali. 3,3-Dimethyl-2-phenyl-1-azirine and perchloric acid in ethyl methyl ketone furnished 2-ethyl-4-phenyl-2,5,5-trimethyl-3-oxazolinium perchlorate. The ring expansion of the same azirine with perchloric acid in acetonitrile yielded the hydrated form of 5-phenyl-2,4,4-trimethyl-4H-isoimidazolium perchlorate, namely, 4-hydroxy-4-phenyl-2,5,5-trimethyl-2-imidazolinium perchlorate, the structure of which was established by spectroscopy, by hydrolysis and hydrogenation products, and by chemical interconversions with 4-ethoxy-4-phenyl-2,5,5-trimethyl-2-imidazolinium perchlorate and 4-isopropoxy-4-phenyl-2,5,5- trimethyl-2-imidazolinium perchlorate, the latter also being made independently by ring enlargement of 3,3-dimethyl- 2-isopropoxy-2-phenylaziridine in acetonitrile with perchloric acid generated in situ. By the use of 15N-labeled acetonitrile in the case of the azirine and mass spectrometric analysis of the product it was possible to determine that the path of incorporation in the final product necessitated cleavage of the 1,3 bond in the azirine. By contrast, the perchloric acid catalyzed methanolysis of 3,3-dimethyl-2-phenyl-1-azirine resulted effectively in 1,2- bond cleavage. Additional ring expansion reactions were realized in the conversion of 3,3-dimethyl-2-methoxy-2- phenyloxirane to 4-methoxy-4-phenyl-2,5,5-trimethyl-2-oxazolinium perchlorate with acetonitrile and perchloric acid and of 3,3-dimethyl-2-isopropoxy-2-phenylaziridine to 5-isopropoxy-5-phenyl-2,2,4,4-tetramethyloxazolidinium perchlorate with acetone and acid. © 1967, American Chemical Society. All rights reserved.