cis,cis-, cis,trans- and trans,trans-o-Dipropenylbenzene (cis,cis-, cis,trans- and trans,trans-1) were prepared. At 225° cis,cis-1 isomerises to give cis,trans-1 and vice versa. The isomerisation follows 1.order kinetics. At equilibrium 89% cis,trans- and 11% cis,cis-1 are present. It is shown by deuterium labelling that the isomerisation is due to aromatic [1,7a] sigmatropic H-shifts. trans,trans-1 rearranges at 225° to yield 2,3-dimethyl-1,2-dihydronaphthalene (3). This can be visualized by disrotatory ring closure of trans,trans-1 followed by an aromatic [1,5s] H-shift. When cis,cis- or cis,trans-1 are heated for 153 hrs a t 225° a small amount (3%) of 1-ethyl-1,2-dihydronaphthalene (5) is formed.