1-cis,2-cis-Dipropenylbenzene (cis,cis-1) isomerises thermally at 215-235° with 1st order kinetics to give trans,cis-1, and vice versa. At equilibrium 89% trans,cis- and 11% cis,cis-1 are present. It is shown by thermal rearrangement of cis,cis-2', 2"-d2-1 that the isomerisation is attributable to aromatic [1,7a]-sigmatropic H-shifts. trans,trans-1 rearranges thermally at 225-245° to yield 2,3-dimethyl-1,2-dihydronaphthalene (2). The formation of 2 can be visualized by disrotatory ring closure followed by an aromatic [1, 5s]-sigmatropic H-shift. 2 is also formed when cis,cis- or trans,cis-1 are heated for 153 h a t 225°. Besides 2 a small amount (3%) of 1-ethyl-1,2-dihydronaphthalene (5) is formed. The rearrangement of trans, trans-1 and trans, trans-2'.2"- d2-1 shows a secondary isotope effect kH/kD = 0.90.