1-tert-Butyl-1,2-cyclooctadiene has been synthesized and found to be stable to dimerization. This molecule is the first example of a kinetically stable allene incorporated into an eight carbon ring;1-tert-Butyl-1,2-cyclooctadiene undergoes a rich variety of fundamental photochemical reactions. Triplet sensitization leads to hydrogen abstraction via planar allene. The site of reaction is highly dependent upon phase. In solution, abstraction occurs from the ring, exclusively at C7, a result in accord with MNDO calculations on the equilibrium triplet state geometry. In the vapor phase, abstraction occurs from the tert-butyl group. No cyclopropylidene derived products are observed in either reaction, a departure from the triplet reactions of other cyclic allenes;The singlet solution phase photoreactions of 1-tert-butyl-1,2-cyclooctadiene involve the intermediacy of a vinylcarbene, 3-tert-butylcyclooct-2-enylidene. Independent generation of this carbene affords the same products as singlet reaction of the allene. This is consistent with results in other systems, where alkyl substitution apparently leads to formation of vinylcarbene intermediates in the singlet reactions of cyclic carbenes.