The thermolysis of 3,3,5,5-tetramethyl-4-methylene-1-pyrazoline (1) proceeds at 1/63 the rate of 4-methylene-1-pyrazoline. The activation parameters, log (k/s−1) = (15.53 ± 0.3) − (40.7 ± 0.4)/θ where θ = 2.303RTin kcal mol−1, suggest that1is undergoing thermolysis by a mechanism different from that for 4-methylene-1-pyrazoline. The 2,2,3,3-tetramethylmethylenecyclopropane (5) produced rapidly isomerizes under the reaction conditions to 2,2-dimethylisopropylidenecyclopropane (4). The four opposed methyl groups of5have created sufficient ground state destabilization as to cause its isomerization to be 147 times faster than the conversion of 2,2-dimethylmethylenecyclopropane to isopropylidenecyclopropane.The products are considered in terms of two possible intermediates, one wherein an allylic diazenyl diradical is considered and the second wherein a Chesick type of intermediate is invoked. For the latter it is demonstrated that substituents on the orthogonal radical center can affect the rotational propensities of methylenevs. isopropylidene of the allylic system.