We have elucidated the origin of the u.v. spectrum given by solutions of styrene, its unsaturated dimer, and its cyclised dimer in 98% sulphuric acid, by working with extremely dilute solutions at low temperatures, and studying the products recoverable from these. No evidence for the existence of the 1-phenethyl ion was obtained, but the 1,3-diphenyl-n-butyl cation has been characterised and an explanation for its being more stable than the 1-phenethyl ion is given. The principal cation formed from styrene and the two dimers is the 3-methyl-1-phenylindanyl cation which has been characterised and which is slightly less stable than the triphenylmethyl ion. The very variable spectrum reported by previous authors as resulting from solution of styrene in concentrated sulphuric acid has been shown to be attributable to the 3-methyl-1-phenylindanyl cation and at least four other aralkyl cations derived from the cyclised dimer, which were characterised by protonation of the corresponding methanol or olefin.The formation of ions from styrene and the two dimers by reaction with excess of perchloric acid in methylene dichloride solution was studied spectroscopically and conductimetrically. From styrene first the unsaturated, and then the cyclised, dimer are formed, and if the excess of acid is not greater than about tenfold, reaction stops at that stage. With a greater excess the 3-methyl-1-phenylindanyl cation and a saturated compound are formed in approximately equal amounts. The kinetics of this ionogenic reaction indicate that it probably involves a complex of the cyclised dimer with perchloric acid reacting with its own kind. (We now know that our earlier views on the formation of ions from styrene and perchloric acid are incorrect.) Among the many ions chatacterised here is the dimethylphenylcarbonium ion.