In the gas phase, in the temperature range 358–399 °C, cis-3a,7a-dihydroindene undergoes two simultaneous reactions which are both homogeneous and kinetically of the first order. The major pathway is an isomerization to allylbenzene. The Arrhenius equation for this reaction is k1= 1014·19 ± 0·09 exp (–219,900 ± 1100 J mole–1/RT) sec.–1. The minor pathway which leads to the formation of indene and hydrogen yields rate constants which fit the Arrhenius equation k2= 1012·77 ± 0·16 exp (–208,900 ± 2000 J mole–1/RT) sec.–1. These results are discussed and tentative conclusions drawn about the transition states involved in the reactions.