The gas-phase unimolecular thermal reactions of the cis and trans isomers of 1-(E)-propenyl-2-methylcyclobutane encompass a complex array of stereomutations, fragmentations, and structural isomerizations. The isomerizations linking 1-(E)-propenyl-2-methylcyclobutanes with (Z)- and (E)-1,5-octadiene and 3-ethylhexa-1,5-diene are governed by a nonhomogeneous system of differential equations which may be solved exactly and used to predict reaction rate profiles for the diene products. Comparisons between these predictions and kinetic data show that the retro-ene reaction of cis-1-(E)-propenyl-2-methylcyclobutane takes place with high stereoselectivity to yield (Z)-1,5-octadiene, which reacts further through Cope rearrangements to afford time-dependent mixtures of the three dienes.