Pyrolysis of bicyclo [2.1.1]hexane in the vapor phase in the temperature range from 327 to 366° led to 1,5-hexadiene as the only detectable product. The kinetics of this isomerization was studied over a pressure range from 0.2 to 20 mm. and found to obey a first-order rate equation under these conditions. The rate of the reaction was unaffected by a 15-fold increase in the surface-to-volume ratio of the reaction vessel or on the addition of nitric oxide or propylene. From the temperature dependence of the process, the first-order rate constant was observed to fit the equation k = 1.49 × 1015 exp(−55,000 ± 1000/RT) sec.-1. The mechanism of the reaction is believed to be analogous to the thermal decomposition of cyclobutane to ethylene. © 1963, American Chemical Society. All rights reserved.