A mixture of 375 parts of phosphoric acid (95% strength by weight) and 1 part of phenothiazine is irradiated for 10 minutes analogously to Example 11. A mixture of 52 parts of styrene, 76 parts of α-methyl-o-chlorostyrene and 94 parts of o-chloro-isopropylbenzene is added to the irradiated solution over 4 hours at 35° to 40° C. After completion of the addition, the mixture is stirred for a further hour at 40° C. The organic phase is separated off and washed with 150 parts of 5 per cent strength by weight sodium hydroxide solution, and fractional distillation gives 20 parts of 1-methyl-3-phenylindan and 68 parts of 1,3-dimethyl-3-(o-chlorophenyl)-indan (b.p. 147° to 152° C (0.4 mm)).
