The kinetics of the thermal rearrangement of [1.1.1]propellane (1) have been investigated by gasphase pyrolysis in a stationary system. The unimolecular reaction leads to dimethylenecyclopropane (2) and its thermal isomerization product ethenylidenecyclopropane (5) with the following Arrhenius parameters: log-(A/s(-1)) = 14.02 +/- 0.23; E-A/kcal mol(-1) = 39.66 +/- 0.52. Furthermore, it was shown that the minor product methylenecyclobutene (3) and its thermal isomerization product 1,2,4-pentatriene (6) result from a heterogeneous side reaction catalyzed by the reaction vessel surface. Ab initio and DFT calculations of the potential energy surface indicate that the isomerization follows an asynchronous reaction path in which two side bonds of the [1.1.1]propellane (1) are involved. The activation barrier at the CCSD(T)/6-311G(2d,p)//MP2/6-311G(2d,p) level of theory was calculated as 40.0 kcal/mol.