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Protonated mesityl oxide and related compounds are characterised by an intense absorption band in the 290 mp region. Evidence is presented to show that, when solutions of t-butyl alcohol or isobutene in acetic acid are mixed with sulphuric acid, condensation with the solvent occurs slowly to give protonated mesityl oxide. The band a t 290 mp found under these conditions is thus a property neither of trimethyl nor of alkenyl cations.ATTEMPTS to study saturated carbonium ions such as the t-butyl cation by their ultraviolet absorption spectra have been confused by coincidences. It has been postulated, on apparently reasonable grounds, that an intense band in the 290 mp region, which is found for solutions of various compounds in sulphuric acid under conditions favouring carbonium ion formation, is characteristic of alkyl cations and, in particular,l of the ion Me&+. Although this postulate has been much q ~o t e d , ~* ~ there are many reasons for caution.

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