The thermal isomerisation of bicyclo[3,1,0]hex-2-ene in the gas phase, in the range 313–347°C, has been found to be homogeneous and first-order. The rate constants for decomposition were independent of pressure in the range 3–30 mm. and fitted the Arrhenius equation, k= 10^14·50 exp(–50,200/RT) sec.^–1. The reaction yields two primary products, cyclohexa-1,3- and -1,4-diene. In the temperature range studied, the 1,3-diene is thermally stable, but the 1,4-diene decomposes to benzene and hydrogen. A brief investigation of the thermal decomposition of cyclohexa-1,4-diene from 303 to 343° showed the reaction to be first-order and predominantly homogeneous; it is probably molecular. The rate constants fitted the Arrhenius equation, k= 10^12·02exp(–42,690/RT) sec.^–1. The decomposition of cyclohexa-1,3-diene occurs at much higher temperatures (above 400°) by a complex process. The mechanisms for the isomerisation of the bicyclohexene and the decompositions of the cyclohexadienes are discussed.